Titanium diboride article and method for preparing same

ABSTRACT

Submicron titanium diboride powder and other hard, refractory metal boride powders, such as zirconium diboride and hafnium diboride powders, are prepared by vapor phase reaction of the corresponding metal halide, e.g., titanium halide, and boron source reactants in the presence of hydrogen in a reaction zone and in the substantial absence of oxygen, either combined or elemental. In a preferred embodiment, the metal halide, e.g., titanium tetrachloride, and boron source, e.g., boron trichloride, reactants are mixed with a hot stream of hydrogen produced by heating hydrogen in a plasma heater. The reaction zone is maintained at metal boride forming temperatures and submicron solid metal boride powder is removed promptly from the reactor and permitted to cool. The preponderant number of metal boride particles comprising the powder product have a particle size in the range of between 0.05 and 0.7 microns. The aforesaid titanium diboride powder can be hot pressed or cold pressed and sintered to articles having densities of at least 90, e.g., 95 percent of theoretical. 
     A metal boride powder product containing a minor concentration of carbon, e.g., from above 0.1 to about 5 percent by weight total carbon can be prepared by adding a source of carbon in the reaction zone. Alternatively, submicron metal carbide powders, e.g., titanium, zirconium, hafnium or boron carbide powders, or finely-divided carbon can be blended physically with the submicron metal boride powder to provide metal borides containing a minor concentration of carbon in the amounts previously indicated. The above described carbon-containing titanium diboride powder compositions can be hot pressed, or cold pressed and sintered to articles having densities of at least 95 percent of theoretical. Such articles can be used as current conducting elements, e.g., cathodes, in electrolytic cells for production of aluminum.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of my copending applicationU.S. Ser. No. 11,596 filed Feb. 12, 1979, now U.S. Pat. No. 4,282,195,which is a continuation-in-part application of my application U.S. Ser.No. 546,838, filed Feb. 3, 1975 now abandoned.

DESCRIPTION OF THE INVENTION

The literature describes a variety of methods for preparing hardrefractory metal borides such as titanium diboride. For example,elemental titanium and boron can be fused together at about 3630° F.This method (synthesis by fusion) produces products that are relativelyimpure and requires isolation of the boride product by chemicaltreatment. Other sintering processes involve the reaction of elementaltitanium with boron carbide (U.S. Pat. No. 2,613,154), the reaction oftitanium hydride with elemental boron (U.S. Pat. No. 2,735,155), and thereaction of ferrotitanium and ferroboron alloys in a molten metalmatrix, e.g., iron (U.S. Pat. No. 3,096,149). A fused salt bathcontaining an alkali metal or alkaline earth metal reducing agent andtitanium-and boron-containing reactants has been used to producetitanium diboride (U.S. Pat. No. 3,520,656). U.S. Pat. No. 3,755,271describes the electrolytic preparation of titanium and zirconiumdiborides by using a molten sodium salt electrolyte and rutile or zirconconcentrates as the source of titanium and zirconium, respectively.

The preparation of the borides of titanium, zirconium, and hafnium bythe vapor phase reaction of the corresponding metal halide, e.g.,titanium tetrachloride, and a boron halide, e.g., boron trichloride orboron tribromide, in the presence of hydrogen at temperatures of from1000°-1330° C., 1700°-2500° C., and 1900°-2700° C., respectively, hasbeen reported in Refractory Hard Metals, by Schwarzkopf and Kieffer, theMacMillian Company, N.Y., 1953, pages 277, 281 and 285. Typically, thesevapor phase reactions have been conducted by heating the reactants inthe presence of an incandescent tungsten filament. Such procedures,however, produce a coating of the metal boride on a heated substraterather than a powdery product. The aforementioned vapor phase reactionfor preparing titanium diboride has been conducted at temperatures lessthan 1200° C. using sodium vapor in lieu of hydrogen (U.S. Pat. No.3,244,482).

A widely reported commercial process used for preparing refractory metalborides, e.g., titanium diboride, is the carbothermic process. In thisprocess, refractory metal oxide, e.g., titanium dioxide, an oxide ofboron, e.g., B₂ O₃, and carbon are heated in an electric arc or highfrequency carbon furnace. As an alternative to the electric arc furnace,it has been proposed to prepare titanium diboride by injecting powderedactivated charcoal impregnated with boron oxide and titania (anatase)into an argon plasma (British Patent Specification No. 1,273,523). Thisprocess describes producing about one gram of product in ten minutes andis not, therefore, considered commercially attractive. The productobtained from the aforementioned carbothermic process is ground in, forexample, jaw-crushers and mills, and screened. To obtain afinely-divided product, extensive milling is required. For example, U.S.Pat. No. 3,052,538 describes the necessity for milling intermetalliccompounds such as titanium diboride and titanium carbide to obtain afine particle size useful for dispersion strengthening of titanium. Amilling time of 300 hours (121/2 days) in a porcelain mill usinghardened steel balls as the grinding medium is recited as beingrequired.

The reported average size of the product produced from such lengthymilling ranges from about 2 to about 10 microns. Moreover, the productis contaminated with impurities abraded from the materials ofconstruction of the mill and grinding surface. Thus, it is common tofind metallic impurities in the product such as tungsten, iron,chromium, cobalt, and nickel. Moreover, extensive milling produces asignificant amount of ultrafine, i.e., less than 0.05 micron, fragments.These fragments are produced during milling and comprise irregularpieces of the principal particles that have been chipped or ground awayfrom the edge or face of the particle. Thus, extensive milling producesparticles having fractured irregular surfaces and a relatively largeamount of fines.

It has now been discovered that submicron refractory metal boridepowder, such as titanium diboride, zirconium diboride and hafniumdiboride powders, can be produced by reacting in the vapor phase, thecorresponding metal halide, e.g., titanium halide, and boron source,e.g., boron hydrie or boron halide, reactants in the presence ofhydrogen, e.g., a hot hydrogen gas stream produced by a hydrogen plasmaheater, and in the substantial absence of oxygen, either combined orelemental. Preferably, hydrogen is heated in a plasma heater to form ahighly heated hydrogen gas stream, which is introduced into the reactorand then into the reaction zone. The metal halide and boron sourcereactants are introduced into the reactor and preferably into the hothydrogen stream and the resulting reactant gas mixture permitted toreact in a zone maintaining at metal boride forming temperatures. Thesolid metal boride formed is removed from the reactor, quenched, usuallyby indirect heat exchange means, and recovered in conventional fineparticle collection equipment, e.g, cyclones, electrostaticprecipitators, dust collectors, etc. The particles of solid, submicrontitanium diboride powder produced by the above process are characterizedby well developed individual crystals that have well developed faces.Substantially all, i.e., at least 90 percent, of the particles have anominal sectional diameter of less than one micron. The preponderantnumber, i.e., greater than 50 percent, of the particles less than onemicron are in the particle size range of between 0.05 and 0.7 microns.The powder product can be produced containing less than 0.25 weightpercent oxygen and less than 0.20 weight percent halogen, e.g.,chlorine.

An intimate mixture of refractory metal boride powder containing carbon,either as free (elemental) carbon or chemically combined carbon, e.g.,as submicron refractory metal carbide, can be produced by introducing avapourous source of carbon into the reactor as a further reactant. Inthis manner, coproduced powders of, for example, titanium diboride andsolid submicron carbon-containing additive, e.g., as elemental carbonand/or titanium carbide in intimate admixture and in most any proportioncan be prepared. Typically, the metal boride powder of this inventioncontains a minor amount of carbon. For use in aluminum reduction orrefining electrolytic cells, consolidated articles prepared from suchrefractory metal boride powder preferably contain between about 0.1 andabout 5 weight percent of total carbon, which is the sum of the carbonpresent in the powder as free carbon and chemically combined carbon. Forother uses, a boride powder product containing higher amounts of totalcarbon can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

The process described herein for preparing submicron refractory metalboride powder, the submicron refractory metal boride particles producedthereby and articles prepared from such powder can be better understoodby reference to the accompanying drawings and photomicrographs wherein:

FIG. 1 is a diagram of an assemblage, partially broken away in section,comprising arc plasma gas heating means, two slot reactant mixer meansfor introducing reactants to the hot gas stream emanating from theplasma heater, reactor means, and auxiliary product recovery equipmentmeans (cyclones and bag filter) for recovering the metal boride powderproduct suspended in the reactor gaseous effluent;

FIG. 2 is a diagrammatic sectional view of the lower portion of the arcplasma gas heating means and upper portion of the reactor of FIG. 1combined with three slot reactant mixer means in place of the two slotreactant mixer means illustrated in FIG. 1;

FIG. 3 is a scanning electron micrograph having a magnification factorof 25,000 of a sample of titanium diboride powder having a B.E.T.surface area of 4.7 square meters per gram that was prepared in a mannersimilar to that described in Example X;

FIG. 4 is a scanning electron micrograph having a magnification factorof 25,000 of a sample of titanium diboride powder having a B.E.T.surface area of 11.5 square meters per gram that was prepared in amanner similar to that described in Example XVIII.

FIG. 5 is a transmission electron micrograph having a magnificationfactor of 25,000 of a sample of the titanium diboride described inconnection with FIG. 4.

FIG. 6 is a scanning electron micrograph having a magnification factorof 3,000, of a sample of purchased titanium diboride powder;

FIG. 7 is a photomicrograph, having a magnification factor of 2100, of apolished etched section of the hot pressed plate prepared in ExampleXII;

FIG. 8 is a photomicrograph, having a magnification factor of 2100, of apolished etched section of the hot pressed plate of Example XI;

FIG. 9 is a photomicrograph, having a magnification factor of 2100, of apolished etched section of the isostatically pressed and sintered rodprepared in Exanple XIV from 7.0 square meters per gram titaniumdiboride; and

FIG. 10 is a photomicrograph, having a magnification factor of 2100, ofa polished etched section of the isostatically pressed and sintered rodof Example XV.

DETAILED DESCRIPTION

Refractory metal borides of Group 4b of the Periodic Table of theElements (Handbook of Chemistry and Physics, 45th edition, published byThe Chemical Rubber Co., 1964) prepared by the process describedhereinafter, namely, titanium diboride, zirconium diboride and hafniumdiboride, are grey to black powders composed predominantly of welldeveloped crystals having well defined faces. FIGS. 3, 4, and 5, whichare electron micrographs (25,000 magnification) of titanium diborideprepared in accordance with the present invention, show examples of thetypical crystalline particles produced. The product contains varyingproportions of equidimensional and tabular single crystals, which arefreely dispersible by virtue of extremely limited crystal intergrowth.The equidimentional crystals are bounded either by planar crystal facesor smooth rounded surfaces. The tabular crystal forms consist dominantlyof hexagonal prisms terminated by the basal pinacord. The tabularcrystals are flattened perpendicular to the c-crystallographic axis as aresult of greater development of the pinacoidal faces relative to theprism faces. Consequently, the crystal habit of the product can bedescribed as tabular to equidimensional hexagonal. Based on visualobservations of the powdery product through an electron microscope, thetabular hexagonal crystals exhibit a nominal sectional diameter tothickness ratio within the range of 1.5:1 to 10:1.

FIG. 6 is a scanning electron micrograph (3,000 magnification) of asample of purchased titanium diboride powder. A comparison of the powderillustrated in FIG. 6 with that of FIGS. 3, 4, and 5 shows clearly thedifference between the purchased titanium diboride powder and thetitanium diboride powder prepared according to the present invention.The purchased powder is significantly larger, as shown by the linearscale on each Figure and their respective magnification factor. Further,the purchased powder does not contain well developed titanium diboridecrystals with well developed faces. The crystals that are present inFIG. 6 are not as well grown as those in FIGS. 3, 4 and 5 and exhibitmany irregular or broken edges. Finally, the presence of a significantamount of ultrafine fragments is evident in the purchased titaniumdiboride product shown in FIG. 6, while there is a substantial absenceof such ultrafine fragments in the product of FIGS. 3, 4, and 5.

Submicron metal boride powders, e.g., titanium diboride, that can beprepared utilizing the process described in more detail hereinafter aresubstantially free of undesirable metal contaminants, i.e., the powdersare essentially pure, as established by emission spectrographicanalysis.

Metal impurities (as elemental metal) normally represent less than 4,000parts per million parts of the boride powder (ppm), i.e., less than 0.4weight percent, and often represent less than 3,000 ppm (0.3 weightpercent). Among the metals that can comprise the aforementionedimpurities are the following: aluminum, barium, calcium, chromium,copper, iron, potassium, lithium, magnesium, magnanese, sodium, nickel,silicon, vanadium and tungsten. The source of such metal impurities, ifpresent, in the boride powder product is normally the reactants orequipment and to prepare the product.

Oxygen and halogen, e.g., chlorine, normally make up the largestindividual non-metallic impurities that are introduced into the productfrom the reactants. By virtue of the described process, it is readilyfeasible to obtain boride powders with less than 0.20 weight percenthalogen, e.g., chlorine, and less than 0.25 weight percent oxygen. Bycareful recovery, e.g., degasification, and handling techniques to avoidexposure of the boride powder product to the atmosphere (oxygen) ormosture, boride powders with less than 0.15, often less than 0.10,weight percent halogen, and less than 0.20, e.g., less than 0.15 weightpercent oxygen can be obtained. The aforementioned values for halogenand oxygen are based upon analysis for such impurities obtained by useof X-ray spectrographic analysis and by the use of a Leco oxygenanalyzer respectively. The aforementioned X-ray spectrographic techniqueanalyzes principally for unreacted metal halides and subhalides presentin the boride powder. Adsorbed hydrogen halide, e.g., hydrogen chloride,on the boride powder may not be detected by that technique.

When not added intentionally, carbon can also be present in the boridepowder product. Normally, the boride powders are substantially free ofcarbon, i.e., the carbon level is typically less than 0.1 weightpercent. Because of its beneficial effect (described hereinfter), carbonis not considered usually as an impurity. Thus, despite the use ofsubstantially pure reactants and careful handing and recoverytechniques, a small amount of metal impurities, halogen, oxygen andcarbon can be present in the product. The total amount of contaminantsin the boride powder product is usually less than 1.0 weight percent,and typically is less than 0.75 weight percent. Stated another way, therefractory metal boride powders of the present process are usually atleast 99 percent pure and typically are at least 99.25 percent pure.

As heretofore indicated, refractory metal boride powders of the presentinvention can be prepared with minor amounts of added carbon. It hasbeen found that carbon notably in the elemental form or as metalcarbide, aids the densification of the boride powder (promotessintering) that is processed by cold pressing and sintering, or hotpressing. As used herein with respect to metal boride powders orcompositions, the term "carbon" or "total carbon" unless otherwisedefined are, intended to mean the carbon present therein both aselemental carbon and chemically combined carbon, e.g., as a metalcarbide.

Consolidated articles prepared from refractory metal boride powderscontaining from about 0.1 to about 5 weight percent total carbon,preferably from above 0.1 to about 2 weight percent, e.g., 0.15 or 0.3to 1 or 2 weight percent, more preferably, about 1 weight percent, totalcarbon based on metal boride are especially useful in aluminum reductionor aluminum refining cells. From the evidence at hand, it is believedthat a total carbon content of at least about 0.3 weight percent willprovide a substantially uniform microstructure in hot pressed or coldpressed and sintered articles prepared from such powder compositions.For other uses, refractory metal boride powders containing higheramounts of total carbon, e.g., up to 10 weight percent or more, arecontemplated. Thus, powder compositions (and articles preparedtherefrom) containing from 0.1 to 10 weight percent total carbon arecontemplated.

The carbon added to the refractory metal boride powder, can be presentas elemental finely-divided carbon or as submicron refractory metalcarbide powder, e.g., hafnium carbide, titanium carbide, tantalumcarbide, zirconium carbide, boron carbide, silicon carbide, etc.Preferably, the refractory metal of the carbide will be the same as theboride; but, identity or refractory metal is not necessary. Thus,compositions such as titanium diboride powders containing carbon ashafnium carbide, tantalum carbide, zirconium carbide, boron carbide orsilicon carbide are contemplated. Other combinations of refractory metalboride powders and refractory metal carbide powders are alsocontemplated. Moreover, while the carbon content of the boride powdercomposition can be introduced by physically mixing the carbon source(carbon or metal carbide) in the amounts desired, it is also possible tointroduce the carbon into the powder composition in the metal boridepowder forming process, i.e., in the reactor and during the vapor phasereaction.

The metal boride powders produced by the present process, e.g., titaniumboride, are, as indicated, predominantly submicron in size. The surfacearea of the boride powder product can vary between about 3 and about 35square meters per gram, (m² /gram). More typically, the surface areawill range between about 3 and about 25 m² /gram or between about 3 or 4and about 15 m² /gram, e.g., between 5 and 10 m² /gram, as measured bythe method of Brunauer, Emmett, and Teller, J. Am. Chem. Soc., 60, 309(1938). This method, which is often referred to as the B.E.T. method,measures the absolute surface area of a material by measuring the amountof gas adsorbed under special conditions of low temperature andpressure. The B.E.T. surface areas reported herein were obtained usingnitrogen as the gas adsorbed and liquid nitrogen temperatures (-196° C.)and a pressure of 150 mm of mercury (0.2 relative pressure).

The surface area of the boride powder is, of course, a function of theparticle size of the boride particles produced, i.e., the smaller theparticle size, the higher the surface area. The average sphericalparticle size diameter, in microns, of the refractory metal boride,e.g., titanium diboride, powder particles produced can be estimatedroughly by the expression:

    Average Spherical Particle Size Diameter=1.33/Surface Area (m.sup.2 /gram)

which assumes that each particle is a sphere (regular shaped polygon).

Using the aforesaid expression, the average spherical particle sizediameter of metal boride powder having the range of surface areasdescribed hereinabove can be calculated. Thus, boride powder of between3 and 35 m² /gram surface area has an average diameter of between about0.4 and about 0.04 microns. Similarly, metal boride powders of between 3and 25 m² /gram, between 3 or 4 and 15 m² /gram and between 5 and 10 m²/gram have average spherical particle size diameters of between 0.4 and0.05, between 0.4 and 0.3 and 0.09, and between about 0.3 and 0.1microns respectively.

Substantially all, i.e., at least 90 percent (by number) of the metalboride particles comprising the boride powder composition are submicron,i.e., have a nominal sectional diameter of less than one micron. Thenominal sectional diameter is the nominal diameter of a particle viewedunder high magnification, e.g., 25,000 magnification, such as seen by anelectron microscope and depicted in electron micrographs. The nominaldiameter is based on the two dimensional surface viewed under highmagnification. The preponderant number, i.e., greater than 50 percent,of the particles less than one micron are in the particle size range ofbetween 0.05 and 0.7 microns. Particles as small as 0.03 microns and aslarge as 2 microns can be present in the powdery product; but, particlesgreater than 2 microns rarely represent more than one percent by numberof the product. The aforesaid crystalline particles less than 0.05microns are distinguishable from the ultrafine fragments less than 0.05microns found in metal diboride powder that has been milled extensively.The metal diboride powders described herein are substantially free offragments less than 0.1 micron, e.g., the ultrafine fragments less than0.05 microns. It is estimated from a study of the refractory metalboride powders of the present invention with a Zeiss TGZ-3 Particle SizeAnalyzer that at least 60 percent on a number basis, more usually atleast 70 percent, e.g., 98 percent, of the boride particles comprisingthe powder are 0.7 microns or less. It is not uncommon to find that theaforesaid percentages represent also the particles within the particlesize range of between 0.05 and 0.7 microns.

It is estimated further that less than 10 percent on a number basis ofthe boride particles are greater than 1 micron. The aforementionedvalues respecting the percentage of boride particles 0.7 microns or lessdepends on the particle size distribution of the powder. Generally, theparticle size distribution is relatively narrow. The number medianparticle size of the boride particles comprising the boride powdercomposition is usually between about 0.08 and about 0.6 micron, moreusually between 0.1 and 0.5 microns, and varies directly with thesurface area of the powder. Because of its high surface area, the metalboride powder tends to adsorb readily oxygen or moisture.

The refractory metal boride powders of the present process are useful asmetallurgical additives, as cermet components, for dispersionstrengthening of metals, as components of the so-called super alloys andnuclear steels, as coatings for materials exposed to molten metals andin refractory applications. When consolidated, those boride powders canbe used as high temperature electrical conductors, as electrodes inmetal manufacture and refining such as aluminum manufacture. Therelatively low electrical resistivities of consolidated shapes preparedfrom these boride powders make them especially desirable as electricalconductors and electrodes. Moreover, it has been found that theelectrical resistivity of hot pressed or cold pressed and sintered formsprepared from the boride powder products, e.g., titanium diboride,produced in accordance with the process described herein are lower thanvalues reported in the literature. For example, electrical resistivityvalues for titanium diboride have been reported as being greater than 10microhm centimeters, e.g., from 10 to 30 microhm centimeters andtypically from 15 to 25 microhm centimeters. In contrast, hot pressed orcold pressed and sintered titanium diboride forms prepared from titaniumdiboride powder produced in accordance with the present invention aretypically less than 10 microhm centimeters, e.g., usually from 5 to 9microhm centimeters at room temperature. The electrical resistivity oftitanium diboride forms of the present invention can be higher than 10microhm centimeters, e.g., 10 to 15 microhm centimeters, withoutaffecting the utility of such forms, e.g., as current conductingelements. The electrical resistivity of zirconium diboride and hafniumdiboride prepared in accordance with the present invention can be alsoless than 10 microhm centimeters at room temperature, e.g., 25° C.

Electrical resistivity can be measured in the conventional manner.Briefly, such measurement is obtained by applying direct current fromtwo electrodes across the specimen to be measured, e.g., a square orrectangular plate, and the potential (voltage) difference between twopoints on the specimen equidistance from the electrodes recorded by anelectrometer. For example, a 2 inch×2 inch×1/2 inch refractory metalboride plate is clamped at the 1/2 inch side between two copperelectrodes and a direct current applied across the plate. A distance of4 centimeters along the line of current flow (2 centimeters on eitherside of the center line) is measured and the end points marked. Theprobes from the electrometer are placed on the end points of themeasured 4 centimeter length and the potential difference measured.Generally, electrical resistivity is taken at 25° C. and the valuesreported in the examples herein were measured at that temperature. Theelectrical resistivity value is calculated from the followingexpression: ##EQU1##

Refractory metal boride powders prepared in accordance with the processdescribed herein can be consolidated into shapes or forms of highdensity by conventional hot pressing, hot isostatic pressing, or coldforming and sintering techniques. The refractory metal boride powders,e.g., titanium diboride of the present invention can be consolidated byhot pressing by subjecting a mold containing the powders to acontinuously applied pressure of from about 0.5 to 50 tons per squareinch, e.g., 1 to 3 tons per square inch, while raising slowly itstemperature to between 1600° C. and 2700° C., e.g., 1800° C.-2500° C.The compacting, heating and subsequent cooling operations are typicallycarried out in an inert atmosphere, e.g., argon or in a vacuum. Theoperation is often carried out in a graphite die having a cavity of theappropriate desired cross-sectional shape. The pressure is preferablyapplied to the powder by plungers acting on opposite ends of the powder,e.g., a column of powder. The nature of the hot pressing process is suchas to render it difficult to form shapes other than flat plates andother relatively simple shapes. Moreover, hot pressing is a relativelyexpensive process and is hard to adapt to large scale production bycontinuous processing.

The refractory metal boride powders of the present invention can beconsolidated by cold forming and sintering by pressing the powder intothe desired shape followed by sintering the resulting form attemperatures between 1800° C. and 2500° C., e.g., 1950° C. and 2250° C.,either in a vacuum or in a neutral (inert) atmosphere. For simple shapessuch as cylinders, plates, or the like, the powders can be dry pressedin matched metal dies. For complicated shapes, slip casting, tapecasting, pressure casting, compression casting, extrusion or injectionmolding can be used to cold form the article. Further, a wax binder canbe incorporated into the powder by techniques such as spray drying,physical blending, etc. and the resulting powder blend molded into thedesired shape in rubber molds. The powder composition can be mixed witha small amount of binder, i.e., from about 1 to 5, e.g., 1, 2, or 3weight percent of paraffin wax dissolved in 1,1,1-trichloroethanesolvent, or by blending the binder and powder at above the melting pointof the binder, e.g., the wax. The solvent is evaporated prior toconsolidating the powder. For injection molding or extrusion up to about18 percent binder may be used. The resulting powder composition-bindermixture can be consolidated by applying pressure to the mixture, e.g.,isostatically or between matched metal dies, either at ambienttemperature or at slightly elevated temperatures, but, significantlyless than sintering temperatures. The pressure applied is in the rangeof 0.5 to 50 tons per square inch, e.g., 2-10 tons per square inch.Alternatively, the powder composition-binder mixture can be extrudedinto the desired shape. The sintering is accomplished by heating theconsolidated shape in vacuum or inert atmosphere at temperatures of from1800° C. to 2500° C., more typically from 1950° C. to 2250° C. Prior tosintering, it may be necessary to first heat the green compacts attemperatures sufficient to remove any organic binder material (if used).Heating at about 200° to 400° C. for about one hour in a vacuum or inertatmosphere is usually sufficient to remove such binder materials. Theterm "cold formed" as used herein means that the metal boride powdercomposition is compacted and shaped, as by pressing or molding, prior tothe sintering operation, as distinguished from hot formed or hot pressedbodies which are shaped and pressed by the application of pressureduring sintering.

According to published reports, cold pressing and sintering ofsubstantially pure refractory metal boride powders, e.g., titaniumdiboride, has not been employed successfully to prepare sinteredarticles of greater than 90 percent of theoretical density. For example,R. A. Alliegro describes sintered titanium diboride and zirconiumdiboride with densities of not greater than about 68 percent and about81 percent of theoretical on page 518 of his article, "Boride andBoride-Steel Cathode Leads", Extractive Metallurgy of Aluminum, Volume1, G. Gerard et al editor, Interscience Publishers, New York, 1962. U.S.Pat. No. 3,028,324 recites that "Current-conducting elements (carbidesand borides of titanium, zirconium, tantalum and niobium) made by theuse of cold pressing techniques . . . possess the disadvantage of havinga relatively high porosity, e.g., up to 20 percent, and of beingpermeable so that the elements can be penetrated by undesirablesubstances . . . " (column 9, lines 5-10). When used in aluminumreduction or refining electrolytic cells, sintered elements with suchhigh porosity levels are susceptible to penetration to the moltenmaterial in the cell, e.g., flux, metal and electrolyte. When suchpenetration occurs, cracking and failure of the sintered elementresults. Consequently, hot pressing has been used to produce productshaving densities near or approaching theoretical densities. Hotpressing, however, limits to a great extent the shape and size ofsintered elements.

It has been found that the substantially pure refractory metal boridepowders, e.g., titanium diboride powder, of the present invention thatcontain less than 0.1 weight percent carbon can be cold pressed andsintered to high densities. For example, such substantially puretitanium diboride powders have been cold pressed and sintered todensities in excess of 90 percent, e.g., 96-98 percent, of thetheoretical density for titanium diboride. Substantially pure refractorymetal boride powder compositions of the present invention containingabove about 0.1 weight percent carbon, e.g., 0.1 to 2 weight percentalso can be cold pressed and sintered to high densities, i.e., at least90 percent of the theoretical density of the refractory metal boride.Depending upon the particular powder of powder composition, densities inexcess of 93 percent of theoretical, e.g., in excess of 95 percent andoften in excess of 98 or 99 percent of theoretical, can be achieved.Stated another way, cold pressed and sintered elements fabricated fromtitanium diboride powder and powder compositions having a porosity levelof not more than 10 percent, often less than 5, e.g., less than 2 or 1,percent now can be obtained. The aforesaid refractory metal boridepowders and powder compositions can be hot pressed to densities at leastequal to that obtained by cold pressing and sintering, i.e., densitiesapproaching the theoretical density. Since the technique of hot pressinglimits to a great extent the shape and size of fabricated shapes, theavailability of cold pressing and sintering as a consolidation techniqueprovides engineering design opportunities which were not possibleearlier.

Examination of the carbon content of sintered, dense titanium diborideshapes prepared from substantially pure titanium diboride powdercontaining less than 0.1 weight percent carbon suggests that a smallamount of carbon is picked up from the furnace environment in which theshapes are sintered, e.g., from the carbon electrodes or from carboncontainers in which the shapes are enclosed, and from the carbonaceousbinder, e.g., wax, that may be used to prepare the green pressed shape.The carbonaceous binder is typically of the type that is easilyvolatilized and thereby removed from the green piece during the initialstages of the sintering cycle. It is estimated that the amount of carbonpicked up from the furnace environment and/or the carbon container isfrom about 0.03 to about 0.06 weight percent. The amount of carbonretained by the sintered shape as a consequence of using a wax binder,e.g., in amounts of about 2 weight percent, is estimated to be fromabout 0.10 to about 0.16 weight percent.

Inspection of the microstructures of dense, sintered titanium diboridearticles by optical microscopy of polished metallographic sectionssuggests that the level of carbon in the article and the sinteringtemperature influences its microstructure. From the evidence at hand itappears that titanium diboride articles prepared by sinteringsubstantially pure titanium diboride powder having less than about 0.1weight percent carbon at about 2050° C., but which articles have morethan 0.1, e.g., 0.13-0.20, weight percent carbon after sintering exhibita microstructure characterized by less and finer porosity that isprimarily intragranular, a finer grain size and a more uniformmicrostructure than titanium diboride articles prepared from suchpowders that have less than 0.1 weight percent carbon in the articleafter sintering. It would appear from this evidence that when the carbonlevel of the sintered article decreases to less than 0.3 weight percent,the amount of porosity in the article increases gradually; and when thecarbon level decreases to less than 0.1 weight percent the pores tend tolocate more readity on the grain boundaries rather than within thegrains, i.e., the pores tend to be intergranular rather thanintragranular. When a sintering temperature of about 2200° C. is used,the microstructure is coarser and may be zoned, i.e., there isfine-grained zone and a coarse-grained zone, the fine-grained zone beingthe exterior (closer to the surface) zone.

The porosity of the aforesaid articles of high density and carbon levelsabove 0.1 weight percent is essentially uniformly distributed throughoutthe article and is substantially submicron in size at sinteringtemperature up to about 2050° C.-2100° C. There is a low levelpopulation of sporadic flaws or larger defects that are commonly foundin consolidated articles. When determined and classified in accordancewith ASTM (American Society for Testing and Material) Method B 276-54for cemented carbides, the apparent porosity of titanium diboridearticles of high density and carbon levels of at least 0.3 weightpercent is typically of Type A-1 or A-2, i.e., apparent porosity sizesunder 10 microns in diameter, with some Type B-1, i.e., apparentporosity size of from 10 to 40 microns in diameter. The apparentporosity of titanium diboride articles of high density and carbon levelsof less than 0.3 weight percent, but more than 0.1 weight percent, e.g.,0.1-0.2 or 0.3 percent, is predominantly of the type A-4, B-1. The Atype porosity can range from A-3 to A-6 and the B type porosity, i.e.,the sporadic flaws, can range from B-0 to B-2. When the carbon level isreduced to about 0.1 weight percent or less, e.g., 0.08 percent, themicrostructure shows increased porosity and larger pores as well as achange in distribution from intragranular to intergranular locations.The term "apparent porosity" is construed to mean (in that ASTM method)all microstructures observed in a photomicrograph of a properly preparedunetched surface, including structures resulting from uncombined carbon,non-metallic inclusions, etc., as well as true, inherent porosity. Atsintering temperatures above about 2100° C., e.g., 2200° C., theapparent porosity of articles of high density and at least 0.3 weightpercent carbon appears to be essentially uniformly distributed and ofthe A-2 type with some B-3 type porosity. The latter type porosity (B-3)may be in part due to plucking of grains during preparation of thespecimen surface. The porosity (especially the submicron porosity)present in the aforesaid articles of high density and carbon levelsabove 0.1 weight percent can be characterized further as substantiallyintragranular and as a closed or discontinuous porosity.

The porosity of titanium diboride articles prepared from purchasedtitanium diboride powder, which is believed to be prepared by thecarbothermic process, is generally not uniformly distributed and istypically larger than the predominantly submicron porosity manifest bythe dense titanium diboride articles of the present invention. Thatporosity can be characterized further as substantially intergranular andas an open or interconnecting porosity. Such porosity is interstitial,i.e., it occurs between the grains. Consequently, the pores tend to beconnected with one another by the grain boundaries.

The apparent grain size, i.e., the average diameter of the refractorymetal boride grain as measured on an etched metallographically polishedsurface, of a refractory metal boride, e.g., titanium diboride, specimensintered up to about 2150° C. and containing from about 0.5 to about 1weight percent total carbon is predominantly fine. As measured onphotomicrographs of the polished surface, the predominant grain size ofthe boride grains of such specimens is generally less than 20 microns,i.e., in the range of about 1 to 20. When sintering temperatures areabout 2000° C., the predominant grain size is in the range of from about1 to 10 microns.

The apparent grain size of a refractory metal boride, e.g., titaniumdiboride, specimen sintered up to about 2100° C., e.g., 2050° C., andcontaining less than 0.3 but more than 0.1, e.g., 0.1-0.2 or 0.3, weightpercent carbon is relatively fine. The predominant grain size of theboride grains of such specimens are predominantly in the range of about2-25 microns. When the sintering temperature is about 2200° C. or thecarbon level of the article is reduced to less than 0.1 weight percent,the grain size appears to be variable and may be zoned, i.e., fine andcoarse grain zones may be discernible in the microstructure.

The apparent grain size varies directly with the sintering temperatureand inversely with the level of total carbon. At sintering temperaturesup to about 2050° C., and total carbon levels of between about 0.5 andabout 1 weight percent, e.g., 0.7 weight percent, the maximum andpredominant grain size of titanium diboride articles range from about 1to about 20 microns. At a sintering temperature of about 2100° C., themaximum grain size for such articles is in the range of 25 to 30 micronswith a predominant grain size of from about 1 to 20 microns. At asintering temperature of 2200° C. the maximum grain size of sucharticles is in the range of 40-50 microns with a predominant grain sizeof from about 5 to 35 microns. At total carbon levels of less than 0.5weight percent, e.g., 0.3 weight percent, the apparent grain size of themicrostructure is slightly larger. For example, at a sinteringtemperature of 2100° C., the maximum grain size is 45 to 50 microns witha predominant grain size of from about 5 to about 35 microns. At asintering temperature of about 2200° C., the maximum grain size is about80 to 85 microns with a predominant grain size of from about 20 to 60microns.

The apparent grain size is commonly reported as a maximum andpredominant range. The maximum range describes the range which includesthe smallest and the largest grains, and includes grains of lowconcentration; while the predominant range includes those grains thatare present only in high concentrations, i.e., the predominant grains.See, for example, ASTM Method B 390-64, which is a description ofpractice for evaluating the apparent grain size and distribution ofcemented tungsten carbide. As used in this description of porosity andgrain size, the term "sintered" or "sintering temperature" is meant torefer also to temperatures used in hot pressing.

The grains of refractory metal boride, e.g., titanium diboride,specimens of the present invention are, except for certainabove-described relatively low carbon compositions, of relativelyuniform size and occur in a microstructure characterized by contiguousgrain boundaries and low porosity resulting in high density of thesintered bodies. The substantial uniformity of the grains results in asintered article having a microstructure that is essentiallyhomogeneous, i.e., a substantially uniform fine microstructure, incontrast with microstructures characterized by fine, medium and coarsegrains distributed throughout the microstructure resulting in anon-uniform or non-homogeneous microstructure.

The shape of the grains of the articles of high density prepared fromthe metal boride, e.g., titanium diboride, powder described herein areessentially equant. Consequently, the internal structure of the articlesshows a mosaic of substantially equidimensional grains with contiguousgrain boundaries. It has been observed that such structures arecharacterized by the substantial absence of linear laminated regions,i.e., lineation or layered structures. Moreover, such structures can becontrasted with one in which elongate lath-shaped grains are predominanta condition which has been observed when titanium diboride articles areprepared with some carbothermically produced titanium diboride powdercompositions.

Titanium diboride articles prepared by sintering (or hot pressing)above-described titanium diboride powder compositions having at least0.3 weight percent carbon at between about 2150° C., and about 2250° C.,e.g., about 2200° C., have been observed to have improved resistance tothermal shock than articles prepared at lower sintering temperatures.Along with the increased grain size evidenced by the microstructure atsuch sintering temperatures, the microstructure appears to includediscernible microcracks along the grain boundaries, i.e., a mosaic ofinterconnected microcracks develops. These microcracks are predominantlyalong the grain boundaries, are estimated to be about the length of thegrains and are largely submicron in width. These microcracks arediscernible on a polished unetched surface of the article at 1500×magnification.

Examination of the microstructure of titanium diboride articles of thepresent invention containing from above 0.1 to about 2, e.g., 0.15 or0.3 to 1, weight percent total carbon and prepared from titaniumdiboride powder compositions in which the carbon-containing additive iscoformed with the titanium diboride, reveals essentially a single phase.Additional phases, e.g., a metal carbide phase, such as titaniumcarbide, are not usually discernible.

Hot pressed or cold pressed and sintered articles having densities ofgreater than 90 percent of theoretical of the refractory metal boridedensity, e.g., at least 92 or 93 percent of theoretical, have beengenerally considered in the art to be impermeable. When refractory metalboride articles of high density, e.g., from 95 to 99+ percent, are usedin, for example, aluminum reduction or refining electrolytic cells, theyare substantially impermeable to the molten material, e.g., moltenaluminum or cryolite electrolyte, to which they are exposed in suchcells. The higher the density, the greater the resistance to penetrationby the aforesaid molten material. The refractory metal boride powdersand powder compositions of the present invention can be fabricated intoarticles having such densities and, accordingly, such articles can beused as current conducting elements in the aforementioned typeelectrolytic cells.

By substantially impermeable is meant that when such articles are usedcathodically in an aluminum reduction cell and exposed to moltenaluminum, the degree of penetration of aluminum through the surface ofthe article is slight, i.e., is less than about 1, e.g., less than about0.5 millimeters after one month's exposure. It has been observed byscanning electron micrograph examination of a post exposure fracturedspecimen that highly dense (99+%), uncracked titanium diboride articlesare penetrated by aluminum to a distance of less than 0.2 millimetersafter one month's exposure as a cathode in an aluminum reduction celland with no observable penetration of cryolite.

The presence of carbon in the boride powder compositions (as free carbonor chemically combined carbon, e.g., as refractory metal carbide)promotes sintering of the boride powder to high densities. While thecarbon can be introduced into the boride powder in any convenientmanner, it is preferred that the carbon be introduced into the powder inthe reactor when the metal boride powder is being formed. Variousadvantages accrue when the carbon is introduced into the boride powderat that time. First, a more homogeneous distribution of carbon in theboride powder product results that can be achieved by physicallyblending. A homogeneous distribution of carbon throughout the boridepowder hinders grain growth during sintering and helps provide a finegrain structure. A fine grain structure generally has greater strengththan a coarse grained structure. Second, elimination of possible oxygenand metal contamination as a consequence of such blending is achieved.Third, the presence of ultrafine carbon particles in the reaction zoneprovides also a source of nuclei which often results in a boride powderproduct of higher surface area than a powder prepared in a reactionsystem that does not have such nuclei. Finally, it has been observedthat less reactor added carbon is required to obtain the same degree ofdensification than is required with physically blended carbon. Resultsobtained with reactor added carbon compare favorably with those obtainedusing twice as much carbon that has been physically blended withpreformed refractory metal boride. It is postulated that the essentiallyhomogeneous dispersion of reactor added carbon throughout the refractorymetal boride powder is a major reason for this result. Further, titaniumdiboride containing reactor added carbon provides a sintered articlehaving an essentially equiaxed grain structure while titanium diboridecontaining physically blended carbon provides a sintered article havingless pronounced equiaxed grains and more elongated grains.

Refractory metal boride compositions comprising mixtures of more thanone metal boride powder are also contemplated herein. Thus blends oftitanium diboride powder with zirconium diboride powder and/or hafniumdiboride powder in most any proportion can be cold pressed and sintered,or hot pressed in the same manner as heretofore described. Such mixturesof boride powders can be prepared by blending the preformed boridepowders in the relative amounts desired; or, the boride powders can beco-produced by introducing into the reactor, usually simultaneously, therefractory metal halides of the metal borides desired and in theproportion desired in the end product. Further, mixtures of the carbidesof the aforementioned refractory metals with such boride powder mixturescan be blended physically with the powder or simultaneously preparedwith the aforementioned refractory metal borides in the amountsdescribed previously by introducing a carbon source into the reactionzone.

Generally, any volatile inorganic titanium, zirconium or hafnium halide,e.g., a compound of only the aforementioned metal and halogen (chlorine,bromine, fluorine and iodine), can be used as the source of theaforementioned metal in the refractory metal boride powder productprepared by the process described herein. As used herein the terms"metal halide" and "metal boride" or "metal diboride" are intended tomean and include the halides and borides respectively of titanium,zirconium and hafnium, i.e., the elements of Group 4b of the aforesaidPeriodic Table of the Elements. However, for the sake of convenience andbrevity, reference will be made sometimes to only one of theaforementioned metal halides or borides.

Exemplary of the refractory metal halides that can be employed in thepresent process include: titanium tetrachloride, titanium tetrabromide,titanium tetraiodide, titanium tetrafluoride, zirconium tetrabromide,zirconium tetrachloride, zirconium tetrafluoride, zirconium tetraiodide,hafnium tetrabromide, hafnium tetrachloride, hafnium tetrafluoride,hafnium tetraiodide, as well as subhalides of titanium and zirconiumsuch as titanium dichloride, titanium trichloride, titanium trifluoride,zirconium dibromide, zirconium tribromide, zirconium dichloride andzirconium trichloride. Of course, subhalides other than the subchloridesand subfluorides can be used in the same manner. Mixtures of metalhalides of the same metal such as the chlorides and the bromides, e.g.,titanium tetrachloride and titanium tetrabromide can be employed as themetal halide reactant. Further, mixtures of halides of different metalscan be used when it is desired to co-produce more than one metal boridepowder, e.g., titanium diboride and zirconium diboride. Preferably, thehalogen portion of the metal halide reactant(s) is the same to avoidseparation and recovery of different hydrogen halides from the productstream. The metal halide reactant(s) can be introduced into the reactantinlet assembly (mixer means) used to introduce the reactants into thereactor as a liquid or vapor; but, should be introduced in such a mannerthat the reactant(s) is a vapor in the reactant mixing zone andsubsequent reaction zone. Economically preferred as the metal halidereactant are the tetrachlorides, e.g., titanium tetrachloride. The metalhalide reactant(s) should be substantially pure, i.e., substantiallyfree of metal contaminants and free or chemically combined oxygen so asto produce a metal boride powder having the purity described earlier.

The boron source reactant like the metal halide reactant should be alsooxygen-free and substantially pure to avoid the introduction of oxygenand metal contaminants into the metal diboride product. By oxygen-freeis meant that the boron source is substantially free of chemicallycombined oxygen, e.g., the oxides of boron, as well an uncombinedoxygen. Despite the precautions of reactant purity, a small amount ofoxygen contamination occurs in the boride powder, as earlier described.As a suitable source of boron for the metal borides, there can bementioned inorganic boron compounds such as boron tribromide, borontriiodide, boron trichloride, boron trifluoride and the hydroborides(boranes), e.g., B₂ H₆, B₅ H₉, B₁₀ H₁₄, and B₆ H₂, Boron trichloride ispreferred. As in the case of the metal halide reactant, the boron sourcereactant is introduced into the reactor in such a manner that it ispresent in the reactant mixing zone and reaction zone as a vapor. Themetal halide source and boron source should be chosen from thosecompounds which, in combination, provide a thermodynamically favorablereaction at the desired reaction temperature. For example, the reactionof titanium tetrachloride with boron trifluoride is thermodynamicallyless favorable at 2000° K. than at 2500° K. Thus, such thermodynamicallyless favorable reactions will require higher reaction temperatures.

The amount of boron source reactant introduced into the reaction zone inthe reactor will be preferably in at least stoichiometric quantities,i.e., in amounts sufficient to provide at least two atoms of boron foreach atom of metal, e.g., titanium, introduced into the reaction zone inthe reactor as metal halide, e.g., titanium halide, reactant. The ratioof the boron source reactant to the metal halide reactant can, ofcourse, vary from stoichiometric quantities. Thus, the boron sourcereactant can be introduced in amounts sufficient to provide in thereaction zone between about 1.8 and about 3 atoms of boron per atom ofmetal, e.g., titanium. Preferably, greater than the stoichiometric ratiois used. For example, the mole ratio of reactants boron trihalide totitanium tetrahalide (BX₃ /TiX₄), wherein X is halogen, can vary fromabout 1.8:1 to 3:1 and preferably is about 2. When a stoichiometricexcess of the boron source is used, less residual unreacted metal halidereactant is found in the product. When a stoichiometric excess of metalhalide is used, sub-halides of the metal are found in the product. Whileit is preferred that the boron source reactant be used in stoichiometricexcess either of the metal halide or boron source reactants can be usedin stoichiometric excess in amounts of from 5 to 30 percent by weight.

In the embodiment wherein carbon-containing metal diboride powder isproduced in the reactor, carbon source reactant is also introduced intothe reaction zone in the reactor. Such carbon source reactant is of thetype that is volatile in the reaction zone and is capable of reacting ina thermodynamically favorable manner at the temperatures at which thereaction is conducted. In the aforesaid embodiment, volatilehydrocarbons, halogenated hydrocarbons or mixtures thereof that aresubstantially pure and oxygen-free, as defined above, can be used as thecarbon source. As used herein, the term "halogenated hydrocarbon", e.g.,"chlorinated hydrocarbon", is intended to mean and include bothcompounds of carbon, halogen and hydrogen, and compounds only of carbonand halogen, e.g., carbon tetrachloride.

Typical hydrocarbons that can be used as the carbon source include thenormally gaseous or liquid but relatively volatile hydrocarbonsincluding saturated and unsaturated C₁ -C₁₂ hydrocarbons, such asmethane, ethane, propane, the butane, the pentanes, decanes, dodecanes,ethylene, propylene, the butylenes and amylenes, symmetricaldimethylethylene and like alkenes, cycloaliphatic and aromatichydrocarbons, such as cyclopentane, cyclohexane, toluene, benzene, etc.,and acetylenic compounds of which may be noted acetylene, methylacetylene, ethyl acetylene, and dimethyl acetylene. Methane or propaneare economically preferred for this purpose. Rarely are hydrocarbons ofmore than twelve carbons used.

Examples of halogenated hydrocarbons that can be used as the source ofcarbon in the process described herein include saturated and unsaturatedcompounds containing from one to twelve, more usually one to eight,carbon atoms, such as methyl chloride, ethyl chloride, chloroform,methylene chloride, carbon tetrachloride, dichlorodifluoromethane, amylchloride, chloroethane, vinyl chloride, 1,1-dichloroethylene,1,2-dichloroethylene, 1,1-dichloroethane, 1,2-dichloroethane, ethylenedibromide, trichloroethylene, perchloroethylene, propylene dichloride,1,1,2-trichloroethane, 1,1,1-trichloroethane, 1,1,1,2- and1,1,2,2-tetrachloroethane, hexachloroethane, and like aliphaticchlorides, fluorides, bromides or iodides containing up to about twelvecarbon atoms, most preferably up to about six carbon atoms. Aromatichalocarbon compounds, e.g., chlorocarbon compounds, also can be used.Such compounds include C₆ -C₉ halogenated aromatic compounds, such asmonochlorobenzene, orthodichlorobenzene, paradichlorobenzene and thelike. Cycloaliphatic halides, such as the C₅ -C₆ aliphatic halides,e.g., chlorinated cyclopentane and cyclohexane, etc., can also be used.

Typically, the above-described hydrocarbons and halogenated hydrocarbonsshould be readily vaporizable (volatile) without tar formation sinceotherwise unnecessary difficulties which are unrelated to the processitself can arise, such as the plugging of transfer lines bydecomposition or polymerization products produced in the course ofvaporizing the carbon source reactant.

The amount of carbon source reactant, e.g., hydrocarbon or halogenatedhydrocarbon, used will of course depend on the amount of carbon desiredin the final boride powder product. The amount of total carbon in themetal diboride powder, e.g., titanium diboride powder, or diboridepowder composition can range from above 0.1 to about 5 weight percent,preferably from above 0.1, e.g., 0.15 or 0.3, to about 1 or 2 weightpercent. When carbon is added to the metal boride powder, total carbonlevels of from about 0.5 to 1 weight percent, e.g., 0.7 weight percenthave been found to be desirable.

When a carbon source reactant is introduced into the reactor, it isexpected that carbide(s) of metal(s) present in the reactor, e.g.,titanium carbide, are co-produced in situ with the metal diboride. Atlow levels of carbon, i.e., less than 1 weight percent total carbon, theX-ray pattern characteristic of metal carbides, such as titaniumcarbide, in the diboride powder is not fairly evident. By "total carbon"is meant the total amount of both free carbon and chemically combinedcarbon, e.g., metal carbide, in the metal diboride powder product. If,for example, all of the co-formed carbon in titanium boride powder ispresent as titanium carbide, a total carbon content of between above 0.1and about 5 weight percent corresponds to a titanium carbide content ofbetween above 0.5 and about 25 weight percent. On the same basis, atotal carbon content of 0.15 to 2, e.g., 1 weight percent, correspondsto a titanium carbide content of between about 0.75 and 10, e.g., 5weight percent.

Metal boride, e.g., titanium diboride, powder containing from above 0.1to 5 weight percent total carbon can be prepared also by blendingphysically submicron metal carbide powder, e.g., titanium carbidepowder, and/or finely-divided carbon with submicron metal boride, e.g.,titanium diboride powder in amounts sufficient to provide a total carbonlevel within the aforesaid range. Submicron titanium carbide and othermetal carbides can be prepared by the process exemplified by U.S. Pat.Nos. 3,485,586, 3,661,523, 3,761,576, and 3,340,020. Generally, thesubmicron titanium carbide used will have a number median particle sizeof between about 0.1 and 0.9 microns. Submicron carbon is commerciallyavailable and such materials can be used directly; a commercial carbonproduct having a particle size larger than desired can be used,preferably by first being reduced in size by grinding the carbon inconventional milling equipment, e.g., fluid energy mills. For example,commercially available N110 carbon black having a surface area of 11-19m² /gram can be used.

The vapor phase reaction of metal halide and boron source reactants withor without a volatile carbon source is conducted in the presence ofhydrogen. The amount of hydrogen utilized in the abovedescribed processis at least that amount which is required stoichiometrically to satisfythe theoretical demand of the reaction. Preferably, the amount ofhydrogen used is in excess of the theoretical amount. When, for example,the metal halide reactant used is titanium tetrachloride and the boronsource reactant used is boron trichloride, the theoretical amount ordemand of hydrogen required can be expressed by the equation:

    TiCl.sub.4 +2 BCl.sub.3 +5 H.sub.2 →TiB.sub.2 +10 HCl I.

Often the amount of hydrogen utilized will be in excess of ten times andas high as 100 times the amount of hydrogen shown to be required by theabove equation or required to equal the chemical equivalents of halogenof the metal halide and/or boron halide reactants. When the boridesource is a hydroboride, the hydrogen available from the hydroboride canbe used to satisfy all or a part of the hydrogen demand. Typically, themole ratio of hydrogen to metal halide reactant ranges between about 20and 40, e.g., 25 moles of hydrogen per mole of metal halide.

The temperature at which the vapor phase reaction of metal halide andboron source reactants is conducted will depend on the reactantsselected and will be those temperatures at which submicron metal boridepowder is produced with the selected reactants under thermodynamicallyfavorable conditions, i.e., metal boride powder forming temperatures.The average reaction zone temperature for the aforementioned vapor phaseproduction of metal boride powder such as titanium diboride powdertypically is above 1000° C. and usually ranges upwardly of 1000° C. toabout 3500° C. The process can be conducted at subatmospheric,atmospheric, and superatmospheric pressures. Typically, the process isconducted at between about 1 and about 3 atmospheres, normally between 1and 1.5 atmospheres pressure.

The process and handling equipment utilized in the aforementionedprocess for producing metal diboride powder (as more specificallydescribed hereinafter) are constructed from materials resistant to thetemperatures and corrosive environment to which they are exposed duringthe various steps of the procedure, as outlined hereinafter. The presentinvention will be more fully understood by reference to the accompanyingdrawings. Referring now to FIG. 1, there is shown apparatus comprisingplasma generator heating means 1 mounted atop reactant inlet assembly(mixer) means 30 which, in turn, is mounted atop reactor 34. Althoughthe aforesaid apparatus is shown in vertical alignment, other alignmentsaway from the vertical including a horizontal alignment arecontemplated. While the plasma generator heating means shown is an archeater, other plasma heater types, e.g., an induction (high frequency)heater, can also be used. Further, other heating means such aselectrical resistance heaters, can be used to heat hydrogen to thetemperatures required by the process described herein. The hydrogen isheated typically to temperatures which is sufficient to establish andmaintain metal boride forming temperatures in the reaction zone bearingin mind that it is mixed with the metal halide and boron sourcereactants which are introduced into the reaction zone at below thereaction temperature, usually significantly below reaction temperatures.Thus, the principal source of heat for the reaction is generally thehighly heated hydrogen gas stream. Plasma heater 1 consists essentiallyof an annular anode 11 which is aligned coaxially with cathode rod 3.Both anode and cathode are mounted in a cylindrical sleeve 9 which iselectrically non-conductive. In the embodiment illustrated, the cathoderod tapers conically at its end essentially to a point. The anode andcathode are constructed out of conventional electrode type materials,such as copper, tungsten, etc. The cathode often has a thoriatedtungsten tip or inserts which assist in cooling of the cathode.

As is conventional with plasma heaters, the anode is surrounded by anannular cooling chamber 13 through which coolant, e.g., water, or othercooling medium is circulated by means (not shown) in order to hold theanode at a suitably low temperature and prevent undue erosion thereof.In a similar manner, the interior of the cathode is provided withcooling chamber 7 and with means (not shown) to circulate water or othersuitable cooling fluid therein in order to hold the cathode at asuitable operating temperature. Tube 2 serves to help support and aligncathode rod 3 and provide a conduit for coolant flow. Cathode 3 can beprovided with means for moving it in a vertical direction so that thedistance between cathode 3 and anode 11 can be varied.

The anode and cathode are axially aligned but spaced longitudinally toprovide annular space 21 which tapers conically to a coaxial outletconduit 23. The assemblage is also provided with plasma or work gasinlet means 15 having conduit 17 which communicates through annularconical conduit 19 with the annular space 21. The cathode and anode areconnected by electrical connecting means (not shown) to a power supply(not shown). Typically, the power source is a direct current powersource.

Reactant mixer means 30 is adjacent to the anode end of cylindricalsleeve 9, and as shown, comprises two coaxial, longitudinally spacedannular conduits 42 and 47 that are provided with inlet nozzle means 40and 45, respectively. As shown, exit port 48 of annular conduit 47 isretracted from exit port 43 of annular conduit 42 to form a conicalreactant introduction zone 24. Reactants from reactant supply means (notshown) are introduced into conduits 42 and 47 through nozzle means 40and 45 respectively. The flow path of the reactants discharged throughexit ports 43 and 48 can be perpendicular to the exiting gas fromconduit 23, as shown. If desired, exit ports 43 and 48 also can bepositioned away from the perpendicular, i.e., downwardly or upwardly, atan angle of from 1° to 45° from the horizontal position shown so thatthe reactant gas flow is directed at such angle into or in contact withthe stream of hot gas emanating from the plasma heater. The reactant gascan be projected radially, tangentially or at any suitable angletherebetween into the downwardly flowing stream of heated plasma gasemanating from outlet conduit 23. The top of reactant mixer means 30contains opening 31 which is coaxially aligned with outlet conduit 23 ofanode 11 to provide an overall direct straight-line path for the heatedplasma gas from plasma generator 1 through reactant mixer means 30 intoreactor 34. Preferably, the heated plasma gas is introduced into thecenter of reactor 34 and spaced from the walls thereof to thereby assistin positioning the reaction zone away from the walls of the reactor.

Typically, hydrogen is used as the gas which is heated by theaforementioned heating means, e.g., plasma heater 1; however, othergases, e.g., the noble gases can be used. Argon and helium are suitableplasma gases. The use of hydrogen as the plasma gas is advantageoussince it insures the establishment of a reducing atmosphere and providesa halogen, e.g., chlorine, acceptor, thereby removing halogen releasedfrom the metal halide, boron halide and/or halocarbon compound reactantsas hydrogen halide. Mixtures of hydrogen with other gases, such as thenoble gases, e.g., argon or helium, can also be employed as the plasmagas. When a noble gas is used as the plasma gas, the hydrogen requiredfor the vapor phase reaction is introduced into the reactor by mixing itwith the reactants, as a part of the boron source reactant, e.g., theboranes, and/or as a separate stream through mixer means 30.

As the heated plasma gas stream moves past the zone of reactantintroduction 24, it mixes with the reactants introduced through reactantmixer means 30. The reactants are introduced usually at below reactiontemperatures. Because of the high heat content of the hot hydrogenstream no special efforts to heat the reactants to temperatures abovewhich they are gaseous are required. The resulting gaseous mixture isforwarded into the interior of reactor 34 and reacted therein. Reactor34 is typically externally water cooled (not shown). Typically, thereactants and reaction mixture are in turbulent flow although laminarflow can be used. The reaction mixture flowing into reactor 34 which isa recirculating-type reactor as opposed to a plug flow-type reactor,typically has an apparent residence time therein of between about 0.05and about 0.5 seconds, more usually between about 0.1 and 0.2 seconds.The apparent residence time can be calculated by dividing the reactorvolume by the gas flow through the reactor.

As shown in FIG. 1, finely-divided metal diboride powder product, whichis suspended in reaction product gases as well as excess reactant gas,hereinafter collectively referred to as product gases or otherequivalent terms, is removed from reactor 34 through conduit 36 andintroduced into cyclones 38 and 39, in order to separate the solid metaldiboride powder from the product gases. The submicron particles of metaldiboride are formed completely in the reactor and since the reactoreffluent is cooled to below metal boride forming temperaturessubstantially immediately, substantially no metal boride formation orindividual particle growth (other than by physical aggregation) occursoutside the reactor. Cyclones 38 and 39 are normally cooled, e.g.,externally water cooled to cool the powder product. For example thecyclones can be traced with tubing through which coolant, e.g., water,is passed. As shown, the discharge from conduit 36 is introducedtangentially into cyclone 38 and from there into cyclone 39 by means ofconduit 51. Titanium diboride powder drops out into receivers 25 and 26,respectively, while gaseous effluent leaves cyclone 39 through conduit52 and into solids separation chamber 28 in which there is disposed abag filter 29, electrostatic precipitator or other convenient means forseparating suspended solids from a gas. Cyclones 38 and 39, andreceivers 25 and 26 are closed to the atmosphere to preventcontamination of the product with oxygen. Thus, the metal diboridepowder that is formed in the reactor at metal diboride formingtemperatures is removed immediately from the reactor and forwarded toproduct collectors that are substantially below temperatures found inthe reactor. The powder product is typically cooled or allowed to coolto room temperature. However, if the cooling capacity of the cyclonesand receivers is not sufficient to provide a powder product at roomtemperature, the product in the receivers may be above room temperature,i.e., from about 20° C. to 100° C., because of the residual heat contentof the powder. Higher temperatures in the receiver may be usedintentionally, as described hereinafter, to promote degassification ofthe powder product. Separation chamber 28 as shown also has an exit orexhaust 50 on the opposite side of the bag filter. As shown, the bagfilter has engaged therewith a suitable shaking means 59 to clear thefilter of metal diboride powder. While only two cyclones and receiversare shown, more than two can be used. Alternatively a single receiverand cyclone can be used. Further, a porous metal filter, as described inU.S. Pat. No. 4,080,431, can be used to collect the product.

Solids separation chamber 28 can also be a caustic water scrubber, oftencontaining packing of some sort, e.g., balls, saddles, etc. for greatercontact. The scrubber separates the fine solids from the gas stream andneutralizes acidic species therein before the gas is discharged to theatmosphere or to a flue. To recover unreacted reactants, hydrogen,hydrogen chloride, etc. from the product gases substantially devoid ofits solids burden, conventional separation and recovery means for suchmaterials can be installed between exit conduit 52 and the flue.Further, if the heat removal from the product recovery apparatus, i.e.,the cyclones and receivers, is insufficient, the product transfer line36 can be externally cooled. Moreover, a cold or cooler compatible gascan be mixed with the exiting product effluent to thereby cool it.

Referring now to FIG. 2, there is shown a partial assembly, incross-section, similar to that of FIG. 1, except that three-slotreactant mixer means 32 instead of two-slot reactant mixer means 30 isshown. In addition to annular conduits 42 and 47, there is shown acoaxial, annular conduit 44 which is spaced longitudinally from annularconduits 42 and 47. The exit port 49 of conduit 44 is retracted fromthat of conduit 47 to further extend conical reactant introduction zone24. Annular conduit 44 is connected to nozzle means 41 for introducingreactant gas into said conduit. Nozzle means 41 is, in turn, connectedwith reactant gas supply means (not shown). Reactant mixer means 30 and32 can be constructed of any suitable material, such as graphite,molybdenum, refractory or any other material which will withstand theheat and corrosive environment present in the reactant introduction zone24. The mixer means can be internally cooled thereby permitting the useof conventional metal fabrication.

In the production of refractory metal boride powders, e.g., titaniumdiboride powder, in the manner described, there is a strong tendency forthe metal diboride powder product to deposit and accumulate on thesurfaces of the reactant mixer means exposed to the reactants. When thisoccurs, the titanium diboride powder can restrict the reactant exitports associated with the reactant mixer means, e.g., exit ports 43 and48 of FIG. 1. Partial blockage of these exit ports upsets the flowpatterns of the reactant and gas streams introduced into reactantintroduction zone 24. Such upset in flow patterns can intensify thegrowth of powder deposits on the exposed surfaces of the reactant mixermeans, such as lip 46 of mixer means 30. The growth of such deposits cancontinue until the reactant exit ports are completely blocked.Significant blockage of such ports affects product conversion and yieldand can cause premature shut down of the process for removal of thedeposits.

Addition of anhydrous hydrogen halide, e.g., hydrogen chloride, toreactant introduction zone 24 as described in U.S. Pat. No. 3,979,500,helps to reduce metal diboride powder deposits on the exposed portionsof the reactant mixer means. Typically, the halogen portion of thehydrogen halide corresponds to the halogen portion of the metal halidereactant. Thus, when titanium tetrachloride is used as the metal halidereactant, the anhydrous hydrogen halide used is hydrogen chloride. Theamount of anhydrous hydrogen halide used can vary; but, typically willrange between about 50 and about 350 mole percent hydrogen halide basedon the metal halide reactant. Any convenient means can be used tointroduce anhydrous hydrogen halide into zone 24. Commonly, theanhydrous hydrogen halide is introduced in admixture with the metalhalide reactant; however, it can be introduced with the boron sourcereactant, the carbon source reactant, if used, or as a separate stream.With reference to FIGS. 1 and 2, it is preferred that the anhydroushydrogen halide be introduced near the top of zone 24.

The metal halide and boron source reactants are mixed commonly with acarrier gas to facilitate their introduction into reactant introductionzone 24. The carrier gas can be hydrogen, recycle hydrogen, recyclesolids-free product gas, or a chemically inert, (i.e., inert withrespect to the reactant with which it is admixed) gas such as the noblegases, e.g., argon and helium. Hydrogen is not used commonly with theboron source reactant, e.g., boron trichloride, for the reason thathydrogen has been observed to react with the boron halide reactantwithin the reactant inlet conduits thereby causing blockage thereof. Theamount of carrier gas used to facilitate the introduction of thereactants can vary; but, generally will range between 250 and 1200 molepercent based on the reactant with which the carrier gas is admixed. Thecarrier gas assists in cooling the mixer means, in keeping reactantconduits free of condensibles and has some effect in controlling themixing of the reactants in zone 24 with a consequent effect on thesurface area of the metal boride powder product.

The mean particle size (and thus surface area) of the refractory metalboride particles comprising the powdery product prepared by the processdescribed herein is a function of many variable within the processsystem some of which can be interrelated. From the evidence at hand somegeneral observations can be made. Particle size tends to increase withan increase in the rate of production. Particle size does not appear tochange significantly with changes in the hydrogen plasma gas flow.Particle size tends to decrease with an increase in the intensity ofmixing resulting from the use of larger amounts of carrier gas (or inertgas) introduced into the reactor other than by means of the plasma gas.Finally, increasing the amount of nuclei from additives, such ashydrocarbons, tends to decrease the particle size.

In carrying out the preparation of refractory metal diboride powder bythe process and with the apparatus described herein, and particularlywith reference to FIG. 1 adapted with reactant mixer means 32 of FIG. 2,a hydrogen-containing gas or noble gas, e.g., argon, is introduced intoplasma generator means 1, through conduit 17 from whence it is directedby means of annular conduit 19, into space 21, between cathode 3 andanode 11. The plasma gas can be introduced in a manner such that the gasflows in a spiral or helical fashion through outlet conduit 23.Alternatively, the plasma gas can be introduced radially into the space21 between the cathode and anode so that there is no helical flowpattern established by the plasma gas and the heated plasma gas exitsthe plasma heater in a substantially linear flow path. When the hotplasma gas moves in a helical pattern, plasma gas collimating means,such as described in U.S. Pat. No. 4,022,872 can be used.

An electric arc is established between the anode and cathode and as thearc passes through the plasma gas, the gas is heated to hightemperatures, usually temperatures above reaction zone temperatures. Ahydrogen-containing plasma gas can have an enthalpy of between 20,000and 60,000 BTU per pound of gas, more commonly between 30,000 and 40,000BTU/pound. The heated plasma gas is projected directly into reactor 34,passed reactant introduction zone 24 formed by the lower lip of anode 11and the exit ports of reactant inlet conduits 42, 47 and 44.

Reactant gases, metal halide and boron source reactant, are introduced,in one embodiment, into nozzles 40 and 41, respectively, and thence intoreactant introduction zone 24 and into the environment of the downwardlyflowing stream of hot plasma gas. The reactant gases can be introducedat a mass velocity such that they are aspirated by the movement of theprojected plasma stream or, they can be introduced into the plasmastream at a mass velocity such that the plasma stream is momentarilyconstricted. Hydrogen can be introduced into nozzle 45 of reactant mixer32 and thence into the reactant introduction zone 24 thereby acting as agas sheath between the metal halide and boron source reactants.

The formation of refractory metal diboride powder by the gas phasereaction of the corresponding metal halide and boron source reactants inthe presence of hydrogen and in the substantial absence of oxygen(combined or elemental) commences essentially immediately with themixing of the reactants in the reaction zone at metal boride formingtemperatures. Optimally, the gas phase reaction is confined to a zonewithin reactor 34 away from the hot surfaces of the reactant mixer meansand the reactor. This minimizes deposition of the metal boride powderproduct on the wall surfaces, which, if not otherwise removed, willcontinue to build-up until causing interruption of the process. Thepowder that builds-up on the walls of the reactor tends to be coarserthan the powdery product removed from the reactor soon after it isformed. Co-mingling the build-up powder on the wall with the principaldiboride powder product contributes to the production of a non-uniformproduct. When the principal powder product becomes non-uniform becauseof coarse powder from the reactor wall the powder product should beclassified to remove oversized particles before being used.

Finely-divided refractory metal diboride powder suspended in reactoreffluent product gas is removed immediately from reactor 34 andintroduced into cyclone 38. A portion of the powder product is removedin cyclone 38 and recovered in receiver 25. Powder product retained inthe gas effluent from cyclone 38 is forwarded via conduit 51 to cyclone39 wherein further amounts of metal diboride powder product are removedand recovered in receiver 26. Additional cyclones and receivers can beused if needed. In most cases, the products from receivers 25 and 26 areblended into a single product.

The reactor effluent product gas, now substantially free of its solidmetal diboride powder content, is forwarded to gas separation chamber 28where it is treated to free it from any remaining suspended metaldiboride powder. As shown, the product gas passes through a bag filter29 and is removed from chamber 28 by means of conduit 50. The productgas now removed of its metal diboride and/or other solids burden can betreated further to recover valuable by-products and remove noxiouscomponents therefrom before being burned or discharged to theatmosphere. If desired, the product gas can be treated to recoverhydrogen and/or hydrogen halide, e.g., hydrogen chloride, for use in thepresent process or in some other process or the cooled product effluentstream can be recycled to the reactor as a source of cooling or diluentgas.

The metal diboride powder product prepared in accordance with theaforementioned process is a finely-divided powder that can adsorb gasessuch as unreacted reactants that may be present in the receiver in whichthe product is collected. To avoid contamination by adsorption,receivers 25 and 26 are heated generally to temperatures above about200° F. (93° C.), e.g., from about 200° F.-600° F. (93° C.-316° C.) toassist in degassing of the product during collection of the product.Simultaneously, it is advantageous to maintain a stream of hydrogen oran inert noble gas, e.g., argon, percolating through the product tofurther assist in the degasification step while the product cools. Ifthe product is not substantially free of unreacted reactants such as themetal halide, e.g., titanium halide, and boron source, e.g., boronhalide, such compounds can react with moisture or oxygen in theatmosphere to form oxides or hydroxides of the metal, e.g., titanium orboron, thereby introducing oxygen contamination into the product.Advantageously, the product is handled without exposure to theatmosphere; however, in some cases, some exposure to the atmospherecannot be prevented. In the event the metal diboride powder productcontains adsorbed chlorine-containing species, e.g., the subhalides ofthe metal halide reactant such as titanium trichloride and titaniumdichloride, such species can be removed by heating the product tobetween about 400° and 1000° C., e.g., 500°-700° C. and preferably about600° C. for between about 1 and 4 hours. In performing such heatingstep, the metal diboride powder is charged to a calciner or similarfurnace, preferably a rotating calciner, and heated to the indicatedtemperatures for the indicated time. A stream of hydrogen or inert gas,such as argon, is maintained over the heated product to help removeundesirable adsorbed gases from the product and prevent exposure tooxygen. After degassing, the boride product can be coated with aparaffin wax or other similar binder material to minimize the rate ofoxygen pick-up during storage and handling.

When a source of carbon is introduced into the reactor to prepare acarbon-containing metal diboride powder (presumably as simultaneouslyproduced metal carbide), the carbon source reactant can be introduced byany convenient means. Thus, the carbon source reactant can be introducedinto the reactor mixed with one or both of the metal halide and boronsource reactants. Alternatively, the carbon source can be introduced asa separate reactant stream. Thus, apparatus such as described and shownin FIG. 2, provides individual conduits for each of the reactants whenthe aforesaid embodiment is used. The reactants can be introduced intothe reactor in any sequence; however, the metal halide, e.g., titaniumhalide, reactant is introduced preferably upstream of the boron sourcereactant. Preferably the carbon source reactant is introduced prior tothe metal halide and boron source reactants. Further, one or more of thereactant gases can be introduced through the same conduit in thereactant mixer means (provided the reactants are at a temperature atwhich inter-reaction does not occur) thereby leaving a conduit for theuse of a sheath gas. Still further, mixer means with four, five or moreslots are contemplated so that each reactant and gas stream introducedthrough said mixer means can be introduced separately.

When it is desired to produce metal boride, e.g., titanium diboride,powder in the absence of co-formed metal carbide, metal halide, e.g.,titanium tetrachloride, reactant can be introduced through the top slotof the three slot mixer means depicted in FIG. 2, hydrogen is introducedthrough the middle slot thereby acting as a sheath gas between the metalhalide reactant and the boron source reactant, e.g., boron trichloride,which is introduced through the bottom slot of the mixer. Alternatively,metal halide can be introduced through the top slot, boron sourcereactant through the middle slot and sheath gas, e.g., hydrogen, throughthe bottom slot. The sheath gas serves to prevent contact of thereactant gases with exposed surfaces of the mixer means 32, such as lip75, and the reactor, e.g., the upper lip 76 of reactor 34. When metalboride, e.g., titanium diboride, is to be produced with co-formed metalcarbide, e.g., titanium carbide, the carbon source reactant can beintroduced through the top slot, the metal halide reactant introducedthrough the middle slot and the boron source reactant introduced throughthe bottom slot. Other reactant introduction sequence can, of course, beused if desired.

The boride powders described herein, particularly titanium and zirconiumdiboride, when hot pressed or cold pressed and sintered into solidshapes of high density are especially useful as current conductingelements in electrolytic cells for the production of metals, e.g.,aluminum. The term "electrolytic cell" as used herein with respect toaluminum production is intended to include both reduction cells andthree-layer cells for the refining or purification of aluminum. Whenused as a current conducting element, titanium and zirconium diboridecan comprise at least part of the cathode of the electrolytic cell or ofthe elements used for conducting electrolyzing current to and/or fromthe electrolytic cell, and can be exposed to the molten metal either inthe reduction cell or in the purification cell.

Both the aluminum reduction cell and the three-layer purification cellare of the type in which electrolyzing current passes through a body ofelectrolyte or flux. In the case of the reduction cell, the currentpasses between an anode and a cathode having their operative faces incontact with the body of electrolyte which has dissolved therein acompound of the metal. The cathode can be the pool of molten metal whichcollects on the floor of the cell or it can be an emersed electrodepresenting a solid surface to the electrolyte. Such an electrode canextend into the pool of molten metal in which case the latter is alsocathodic. In the case of three-layer aluminum purification cells, thecurrent passes between the pool of aluminum alloy forming the bottomlayer in the cell and the layer of purified molten aluminum forming thetop layer in such a cell through the body of electrolyte or flux formingthe intermediate layer which is in contact with both the top and bottomlayers. The operative face or faces of the current-conducting element,i.e., the face or faces exposed to the deleterious conditions subsistingduring the operation of the electrolytic cell, e.g., the face or facesexposed to the molten metal, can be fabricated from the metal diboride,e.g., titanium and zirconium diborides described herein.

Currently, carbon is used extensively for the construction ofcurrent-conducting elements in aluminum reduction electrolytic cells.However, the use of carbon entails a number of very considerablydisadvantages, not the least of which is the fact that the floor of thecell lining which supports the molten metal must, in practice, bearranged in a substantially horizontal plane. With such arrangement, thefloor space occupied by a single cell is quite extensive and the cost ofconstructing such large cells is considerable. The necessity for thehorizontal arrangement arises from the fact that molten aluminum doesnot wet carbon. Further, the gradual penetration of molten flux or fluxconstituents into the cell floor causes the carbon floor to swell ordisintegrate and shortens its useful life. Deposits are formed on thesurface of the carbon which increase the voltage drop across the celland reduce the efficiency of the latter. Still further, the horizontalconstruction has the further disadvantage that the inherent turbulenceof the molten metal cathode requires a high inter-polar distance toinsure against contact of the molten metal cathode with the anode andwith the consequent production of excess heat which has to bedissipated.

Current conducting elements prepared with metal diboride powders of thepresent invention can be disposed in a vertical or inclined position inthe electrolytic cell for the reason that molten aluminum wets thesurface of such elements. Thus, a cathode prepared from the titaniumdiboride powder of the present invention can be arranged in theelectrolytic cell so that the operative face or faces of the cathode aredisposed at a relatively large angle, i.e., 60 or 90 degrees, to thehorizontal, thereby allowing the deposited aluminum to continuouslydrain from the face or faces of the cathode and preferably to collect ina pool in contact with a lower part of the cathode from which pool itmay be withdrawn from time to time in the usual manner. Due to theinclined or substantially vertical arrangement of the cathode, the floorspace occupied by the electrolytic cell is very considerably reduced inrelation to that which is conventionally required. Perhaps the largestadvantage to the use of inclined or substantially vertically arrangedelectrodes of the instant metal diborides is that surging of the moltenaluminum is less likely to occur so that the spacing of the anode andcathode can be substantially reduced compared with that adopted inaluminum reduction cells heretofore known and the dissipation ofelectrical energy in the electrolyte correspondingly reduced. Moreover,current conducting elements prepared from titanium diboride compositionshave relatively high electrical conductivity, i.e., a low electricalresistivity, and therefore the voltage drop due to the passage of theoperating current is less than that experienced in cells of orthodoxconstruction. The effect of sludge formation at the bottom of the cellwhich causes an undesirable additional voltage drop at the cathode inexisting horizontal cells can also be avoided. Thus, the use ofcurrent-conducting elements prepared from metal diboride powder of thepresent invention in aluminum reduction cells improves the passage ofelectrolyzing current through the cell because of the low electricalresistivity of the compositions, and further, when such elements areused in a substantially vertical or inclined position, the voltage dropacross the electrolytic cell is significantly reduced thereby providingsignificant savings in power. Such power savings have becomeincreasingly more important due to the continuing rising cost of power.

The use of titanium diboride current-conducting elements in electrolyticcells for the production or refining of aluminum is described in thefollowing U.S. Pat. Nos. 2,915,442, 3,028,324, 3,215,615, 3,314,876,3,330,756, 3,156,639, 3,274,093 and 3,400,061. Despite the ratherextensive effort these patents indicate was mounted and the potentialadvantages for using titanium diboride and titanium diboridecompositions as current-conducting elements in electrolytic cells forthe production of aluminum as described in the aforementioned patents,such compositions do not appear to have been commercially adopted on anysignificant scale by the aluminum industry. The reasons for such lack ofacceptance are believed to be related to the lack of stability of thecurrent-conducting elements prepared from the titanium diboride powdersof the prior art during service in electrolytic reduction cells. It hasbeen reported that such current-conducting elements prepared withcompositions of the prior art fail after relatively short periods inservice. Such failure has been associated in the past with penetrationof the current-conducting element structure by the electrolyte, e.g.,cryolite, thereby causing critical weakening of the self-bondedstructure with consequent cracking and failure. Other reasons proposedhave been the solubility of the compositions in molten aluminum, moltenflux or electrolyte, or the lack of mechanical strength and resistanceto thermal shock.

Ideally, a current-conducting element should have the followingcharacteristics:

1. Good electrical conductivity.

2. It must not react with nor be soluble in either molten aluminum or,under cathodic conditions, in molten flux or electrolyte, at least toany appreciable extent at the operating temperature of the cell. Thesolubility of the material in molten aluminum is an importantconsideration as it determines both the useful life of thecurrent-conducting element and the degree of contamination of thealuminum produced through the agency of such current-conducting element.

3. Wetability by molten aluminum.

4. Capable of being produced and fabricated into required shapeseconomically.

5. High stability under the conditions existing at the cathode of thecell, i.e., it should possess good resistance to penetration by themolten electrolyte (cryolite) and to cracking.

6. Low thermal conductivity.

7. Good mechanical strength and resistance to thermal shock.

In order to have high stability under service conditions and resistanceto penetration by the electrolyte, the current-conducting elementprepared of titanium diboride powder compositions must have a relativelyhigh density. In the past, high densities have been achieved with metalboride powder compositions of the prior art by hot pressing only. Themetal boride powders of the present invention can be cold-formed andsintered to high densities. These metal boride powders provide theopportunity for preparing current-conducting elements of simple andcomplex shapes at a reasonable cost. Such current-conducting elementsare resistant to the environmental existing in electrolytic cells forthe reduction or purification of aluminum and have improved stabilitycompared to prior art boride compositions in such electrolytic cells.

The present process is more particularly described in the followingexamples which are intended as illustrative only since numerousmodifications and variations therein will be apparent to those skilledin the art. In the following examples, some volumes of gas are expressedin cubic feet per hour at standard conditions [14.7 pounds per squareinch (101.3 kPa) pressure and 70° F. (21° C.)] or SCFH. Reactant andother gas stream rates were measured at nominal laboratory conditions,i.e., 1 atmosphere and 70° F. (21° C.), and are reported as measured ifother than SCFH. Unless otherwise specified all percentages are byweight.

The following examples illustrate the preparation of refractory metalcarbides by vapor phase reaction of the corresponding metal halide and aboron source in the presence of a hot hydrogen stream and in thesubstantial absence of oxygen, combined or elemental.

EXAMPLE I

A heated gas stream was produced by supplying an argon-hydrogen gasmixture, which comprised 95.6 SCFH of argon and 29.9 SCFH hydrogen, toan induction plasma heater. The plasma heater was a quartz tubesurrounded by a cooling jacket, which was provided with an inlet andoutlet through which water coolant was passed. The quartz tube had anaxial hollow core through which the work gas was passed. Surrounding thequartz tube was a 5-turn externally cooled copper coil energized by a 25kilowatt Toccotron radio frequency generator, operating at about 4.5megahertz. The radio frequency (R.F.) power level supplied to theinduction coil was 18.5 kilowatts. The argon-hydrogen hot gas producedby the induction plasma heater was calculated to have an enthalpy ofabout 336 BTU/feet³.

The plasma heater was mounted atop a copper cylinder which served as thereactor. The reactor was also externally cooled. Between the bottom ofthe plasma heater and the top of the copper reactor was positionedreactant inlet mixer means containing four injection ports which weredisposed in a horizontal plane and positioned 90 degrees from eachother. Gaseous titanium tetrachloride at a rate of 2.8 grams/minute(measured at 59° C.), together with 11.8 SCFH of argon as a carrier gaswas supplied to two of the injection ports located 180 degrees apart.Into the other spatially opposed injection ports was introduced 841cc/minute of gaseous boron trichloride together with one liter perminute of hydrogen as a carrier gas. This represented about a 20 percentstoichiometric excess of boron trichloride based on the production oftitanium diboride (TiB₂). The reaction was conducted for about 200minutes.

Reactor product was passed through an externally water cooled coppertube to cool the powder product to a point at which it could becollected in a Teflon bag filter. The product obtained wasfinely-divided and had a surface area of about 13.0 square meters pergram. X-ray diffraction analysis of the product showed it to be titaniumdiboride (TiB₂). The product fumed when exposed to air indicating thatthe material was pyrophoric.

EXAMPLE II Run A

The procedure of Example I was repeated except that the R.F. power tothe plasma heater was about 21 kilowatts and the plasma gas was 120.2SCFH of argon. Seven hundred eighty (780) cc./minutes of borontrichloride together with one liter per minute of hydrogen as thecarrier gas and 11.8 SCFH of argon carrier gas with titaniumtetrachloride reactant were introduced into the heated gas emanatingfrom the plasma heater. The aforementioned amount of boron trichloriderepresented an 8.0 percent stoichiometric excess. The run was continuedfor 3 hours and the powder product recovered. Analysis of the productshowed it to be titanium diboride. The product removed from the reactorwas pyrophoric.

Run B

The procedure of run A was repeated except that 720 cc./minute of borontrichloride were introduced as the reactant and the plasma gas comprised86 SCFH of argon and 32.5 SCFH of hydrogen. The run was continued for 3hours and the recovered powder product found to have a B.E.T. surfacearea of 12.0 square meters per gram. The product was identified astitanium diboride and as removed from the reactor found to bepyrophoric.

Run C

The procedure of run B was repeated except that the R.F. power to theplasma heater was about 20 kilowatts, the plasma gas rate was 86 SCFH ofargon and 32.5 SCFH of hydrogen, and the boron trichloride reactant feedrate was 650 cc./minute. The run was continued for 146 minutes and therecovered powdered product identified as titanium diboride. The producthad a B.E.T. surface area of 10.3 square meters per gram. The productremoved from the reactor was observed to be pyrophoric.

EXAMPLE III

The apparatus of Example I was modified by substituting for the reactantinjection ports, reactant inlet assembly means similar to assembly means30 of FIG. 1 of the attached drawings. The R.F. power to the plasmaheater was about 24 kilowatts; the hydrogen-argon plasma gas was 78.4SCFH argon and 42.7 SCFH hydrogen. Eight hundred forty-three (843)cc./minute of gaseous boron trichloride, 8.59 grams/minute of gaseoustitanium tetrachloride, and 1.9 SCFH of argon carrier gas wereintroduced through the bottom slot of the reactant inlet assembly meansand 38 SCFH of argon shroud gas were introduced through the top slot ofthe reactant inlet assembly means. The run was continued for 200minutes. The powder product was recovered in cyclone receivers and foundto have a B.E.T. surface area of 9.3 square meters per gram. The productwas observed to be pyrophoric.

EXAMPLE IV

Apparatus similar to FIG. 1 of the attached drawing was utilized toprepare titanium diboride. The arc heater utilized was a medium voltage,medium amperage heater having a power input of 28 kilowatts. This heaterhas been operated with an efficiency of between 50 and 60 percent. Thereactant inlet assembly means used was one in which the reactants wereintroduced into the hot gas emanating from the plasma heater from portsdisposed horizontally 180 degrees apart. Hydrogen at a rate of 19 litersper minute was introduced into the plasma heater and heated therein. Theamount of power supplied to the plasma heater was 7.2 kilowatts.Titanium tetrachloride at a rate of 0.99 grams per minute, together with3.6 liters per minute of argon as the carrier gas and 0.25 liters perminute of boron trichloride together with 3.0 liters per minute of argonas the carrier gas was introduced into the hot hydrogen stream emanatingfrom the plasma heater. The run was continued for 60 minutes producingtitanium diboride product at a 98.6 percent yield. The recovered productwas identified as titanium diboride by X-ray diffraction analysis. Theproduct fumed when exposed to air, indicating that the product waspyrophoric.

EXAMPLE V Run A

Apparatus similar to FIG. 1 was used to prepare titanium diboride. Thearc heater was a medium voltage, medium amperage heater having a powerinput of 28 kilowatts. The arc heater was operated at between 24-28kilowatts. Hydrogen in the amount of 300 SCFH was introduced into thearc heater as the plasma gas. Gaseous titanium tetrachloride in theamount of 18.7 grams per minute, together with hydrogen as the carriergas in the amount of 20 SCFH, was introduced through the top slot of thereactant inlet assembly means. Gaseous boron trichloride, in the amountof 26.9 grams per minute with an argon carrier gas in the amount of 22SCFH was introduced through the bottom slot of the assembly means. Therun continued for 951/2 minutes and titanium diboride having a B.E.T.surface area of about 14 square meters per gram was obtained. Titaniumdiboride deposits on the bottom lip of the reactant inlet assembly wereobserved at the end of the run.

Run B

The procedure of Run A was repeated except that boron trichloride wasintroduced through the top slot and titanium tetrachloride through thebottom slot of the reactant inlet assembly means. 25.6 Grams per minuteof gaseous boron trichloride with 22 SCFH argon and 18.7 grams perminute of titanium tetrachloride together with 12 SCFH of hydrogenchloride were utilized as the reactants. The run was continued for 120minutes to produce titanium diboride, having a B.E.T. surface area ofabout 9.1 square meters per gram. A thin skin of titanium diboridepowder deposits on the inlet assembly were observed at the end of therun. Most of the deposit was found to be attached to the bottom exposedportion of the inlet assembly, e.g., lip 46 of mixer means 30 in FIG. 1,and the exposed top lip of reactor 34.

Run C

The procedure of Run A was repeated, except that 12 SCFH of hydrogenchloride was utilized as the carrier gas for the titanium tetrachlorideinstead of the 20 SCFH of hydrogen and 27.8 grams per minute of borontrichloride was fed to the reactor. This run continued for 150 minutesand the titanium diboride product was found to have a B.E.T. surfacearea of about 5.8 square meters per gram. No growth of titanium diboridedeposits on the inlet assembly means was observed.

Run D

The procedure of Run C was repeated, except that the titaniumtetrachloride feed rate averaged about 21 grams per minute and the borontrichloride feed rate averaged about 29.8 grams per minute. This runcontinued for 975 minutes and the titanium diboride product had a B.E.T.surface area of about 6.3 square meters per gram. No growth of titaniumdiboride deposits on the reactant inlet assembly means was observed atthe end of the run.

In all of the above examples, the powder product obtained was calcinedin the presence of hydrogen at 1000° C. to degasify the product. Some ofthe calcined products remained pyrophoric.

Run E

Run D was repeated except that 23 SCFH of hydrogen was added to thetitanium tetrachloride reactant introduced through the top slot of thereactant inlet assembly means. The titanium tetrachloride and hydrogenchloride reactant addition rates averaged 19.2 grams per minute and 2.5SCFH, respectively. Boron trichloride in the amount of 27.0 grams perminute together with 22 SCFH argon was introduced through the bottomslot of the reactant inlet assembly means. This run continued for 1,072minutes and produced titanium diboride having a B.E.T. surface area ofabout 14.1 square meters per gram.

EXAMPLE VI

Apparatus analogous to FIG. 1 modified with the reactant inlet assemblymeans of FIG. 2 was used to prepare finely-divided titanium diboride.The power to the plasma heater was 22.5 kilowatts. Hydrogen in theamount of 300 SCFH was used as the plasma gas. 0.71 grams per minute of1,1,2-trichloroethane together with 45 SCFH hydrogen as a carrier gaswas introduced through the top slot of the three-slot reactant inletassembly means, which was fabricated from graphite. Titaniumtetrachloride in the amount of 18.8 grams per minute together with 20SCFH hydrogen and 5 SCFH hydrogen chloride was introduced through themiddle slot of the reactant inlet assembly means. Boron trichloride inthe amount of 21.7 grams per minute, together with 22 SCFH argon wasintroduced through the bottom slot of the reactant inlet assembly means.This run was continued for 989 minutes and produced titanium diboridehaving a B.E.T. surface area of 24.0 square meters per gram. The productwas analyzed for carbon and found to have 0.55 percent total carbon.

Examples I-VI show that submicron titanium diboride having a surfacearea of between about 3 and about 35 square meters per gram, moretypically between about 4 and about 15 square meters per gram, can beproduced by the vapor phase reaction of titanium halide and a boronsource compound. The submicron titanium diboride powder are well formed,individual crystals of titanium diboride. Typical scanning andtransmission electron micrographs of such titanium diboride is shown inFIGS. 3, 4 and 5 which are described in more detail hereinbefore. Acomparison of the photomicrograph of FIGS. 3 and 4 with that of FIG. 6,which is a scanning electron micrograph of a sample of purchasedtitanium diboride powder, clearly shows the difference between the twoproducts. The photomicrographs of FIG. 6 illustrates a product withill-defined crystals, irregular faces, agglomerated product and asignificant amount of fines, which are apparently produced by milling aproduct that was originally larger in size.

EXAMPLE VII

Apparatus similar to FIG. 1, which is described in Example V Run A, wasused to prepare zirconium diboride. Hydrogen in the amount of 300 SCFHwas introduced into and heated by the medium voltage, medium amperagearc heater. Gaseous zirconium tetrachloride, at a rate of 20.5grams/minute, and 100 SCFH argon were introduced through the bottom slotof the reaction inlet assembly into the hot hydrogen stream emanatingfrom the arc heater. Gaseous boron trichloride, at a rate of 4.93liters/minute (a 25 percent stoichiometric excess based on zirconiumtetrachloride), and 22 SCFH of argon were introduced through the topslot of the reactant inlet assembly. The process was continued for 42minutes. The zirconium diboride product recovered had a B.E.T. surfacearea of 7.7 square meters per gram.

The procedure and apparatus of Example VII can be used to preparefinely-divided, submicron hafnium diboride by substituting hafniumtetrachloride for the zirconium tetrachloride of Example VII. A productsimilar in size and surface area of the zirconium diboride of ExampleVII can be expected.

EXAMPLE VIII

Apparatus analogous to that used in Example VI was used to preparetitanium diboride. 300 SCFH of hydrogen was used as the plasma gas.Propane (89 standard cc/minute), and 45 SCFH hydrogen as a carrier gaswere introduced into the reactor through the top slot of the three-slotreactant inlet assembly means. Titanium tetrachloride (52 grams/minute)together with 9 SCFH of hydrogen and 24 SCFH of hydrogen chloride wereintroduced through the middle slot, and boron trichloride (13,000standard cc./minute) and 22 SCFH of argon were introduced through thebottom slot of the inlet assembly. Titanium diboride powder product wasrecovered and degassed under a hydrogen flow of 11 SCFH at 600° C. for43/4 hours. The titanium diboride powder product had an elementalanalysis of 31.9 percent boron, 0.09 percent oxygen, 0.78 percent carbonand 0.088 percent chlorine, and had a B.E.T. surface area of about 6.4m² /gram.

EXAMPLE IX

Apparatus analogous to that used in Example IX was used to preparetitanium diboride. 300 SCFH of hydrogen was used as the plasma gas.Titanium tetrachloride in the amount of about 41.5 grams/minute, 9 SCFHof hydrogen and 24 SCFH of hydrogen chloride were introduced into thereactor through the top slot of the three-slot reactant inlet assemblymeans. About 22 SCFH of hydrogen was introduced through the middle slot;and, boron trichloride in the amount of about 10,700 standard cc./minute(about a 10 percent stoichiometric excess) and about 22 SCFH of argonwere introduced through the bottom slot of the inlet assembly. Titaniumdiboride powder was recovered and degassed under hydrogen at 600° C. for3 hours. The titanium diboride powder product had an elemental analysisof 32.3 percent boron, 0.44 percent oxygen and 0.03 percent chlorine,and had a B.E.T. surface area of 3.3 m² /gram.

EXAMPLE X

The procedure of Example X was repeated and titanium diboride powderhaving an elemental analysis of 32.3 percent boron, 0.60 percent oxygenand 0.10 percent chlorine was recovered. The product had a B.E.T.surface area of 4.5 m² /gram.

The following examples illustrate the utility of the refractory metalborides.

EXAMPLE XI

A portion of the titanium diboride powder of Example IX was hot pressedat about 2100° C. and 3500 pounds per square inch into a plate 2inches×2 inches×1/2 inch. The plate had a density of 97 percent of thetheoretical density of TiB₂ and a resistivity of about 7 microohmcentimeters. The plate was analyzed for oxygen, which was found to beabout 0.05 percent. The plate was operated as a cathode in an aluminumreduction cell for 100 hours at 960° C. at an anode current density of6.5 amperes/inch². At the end of the test period, the plate was removed,fractured, and inspected. No deterioration of the plate and nopenetration of electrolyte into the plate was observed. Fracture of theplate was observed to be primarily transgranular.

A piece of the test plate was cut out after the test was completed andpolished and etched. FIG. 8 is a photomicrograph, having a magnificationfactor of 2100, of a polished and etched section of the plate. Themicrostructure of FIG. 8 shows a mosaic of equidimensional TiB₂ grainswith contiguous grain boundaries and a limited grain size range. TheTiB₂ grains range from about one to fifteen microns in diameter; but,are predominantly in the four to twelve micron range in size. Titaniumcarbide occurs as occlusions less than one micron in size within thetitanium diboride grains.

EXAMPLE XII

A blend of the titanium diboride powders of Examples X and XI in aweight ratio of about 58.5/41.5 was mixed with about 5 weight percent oftitanium carbide powder having a B.E.T. surface area of about 4.5 m²/gram. The titanium carbide powder was prepared in accordance with theprocedures described in U.S. Pat. No. 3,485,586. The titanium diborideand titanium carbide powders were mixed with 1 percent paraffin wax in1,1,1-trichloroethane with a high speed Cowles mixer. The blendedmixture was vacuum dried and hot pressed at about 2000° C. and 3500pounds per square inch into a 2 inch×2 inch×1/2 inch plate. The platewas allowed to cool overnight in the mold under vacuum. The plate had adensity of about 93 percent of the theoretical density of TiB₂ and wasfound to have an oxygen content of about 0.33 percent. The electricalresistivity of the plate was 6 microohm centimeters. The plate wasoperated as a cathode in an aluminum reduction cell under the sameconditions as recited in Example XII. At the end of the test period, theplate was removed, fractured and inspected. Some minor spalling anderosion of the plate had taken place; but, no penetration of theelectrolyte into the plate was observed. Fracture of the plate wasobserved to be primarily transgranular.

A piece of the test plate was cut out after the test was completed andpolished and etched. FIG. 7 is a photomicrograph, having a magnificationfactor of 2100 of a polished and etched section of the plate. Themicrostructure of FIG. 7 is fine and shows interlocking grains of white,lath-shaped TiB₂ with grey TiC grains dispersed in the structure. TheTiB₂ grains range in size from less than one micron to five microns. TiCgrains are up to three microns in diameter.

EXAMPLE XIII

A blend of 95 parts of titanium diboride powder prepared in a mannersimilar to Example V Run E and 5 parts of titanium carbide powder wasmixed with about 1 percent paraffin wax in 1,1,1-trichloroethane andball milled for about one hour. The titanium diboride powder had aB.E.T. surface area of 4.9 m² /gram and the titanium carbide powder hada B.E.T. surface area of about 5.0 m² /gram. The blended mixture wasvacuum dried and isostatically pressed at about 20,000 pounds per squareinch into a cylindrical rod 11/2 inch in diameter×163/8 inches long. Awell 3/8 inch in diameter and about 15 inches deep was drilled out ofthe rod and the resulting rod was vacuum sintered at about 1900° C. forabout 1 hour. The rod had a density of 95 percent of the theoreticaldensity of TiB₂. The sintered rod was tested as a thermocouple well inan aluminum reduction cell. The rod showed excellent thermal shockresistance and resistance to the bath.

EXAMPLE XIV

Rods similar to that of Example XIV were prepared using titaniumdiboride powder having B.E.T. surface areas of 6.6 m² /gram and 7.0 m²/gram. The sintered rods had densities of 96 percent and greater than 99percent of the theoretical density of TiB₂ respectively. A piece of therod prepared with the 7.0 m² /gram titanium diboride was polished andetched. FIG. 9 is a photomicrograph, having a magnification factor of2100, of a polished and etched section of the rod. The microstructure ofFIG. 9 shows a mosaic of relatively equidimensional TiB₂ grains with thelight-grey TiC predominatly localized in interstices between TiB₂ grainsor occurring as occlusions within the TiB₂ grains. Electron microprobeanalysis has indicated that a gold color induced in the TiC signifiesscavenging of oxygen and nitrogen to produce a solid solution phaserepresented by Ti (C,O,N).

EXAMPLE XV

Titanium diboride powder prepared in a manner similar to Example IX andhaving a B.E.T. surface area of 24 m² /gram and 0.46 percent carbon wasisostatically pressed at 20,000 pounds per square inch into acylindrical rod. The rod was vacuum sintered at about 2000° C. for about30 minutes. The sintered rod, which had dimensions of about 1 inch×5inches, had a density of about 98 percent of the theoretical density ofTiB₂ and a resistivity of about 9 microohm centimeters.

A piece of the rod was polished and etched. FIG. 10 is aphotomicrograph, having a magnification factor of 2100, of a polishedand etched section of the rod. The microstructure of FIG. 10 shows amosaic of equidimensional TiB₂ grains with contiguous grain boundariesand a limited grain size range. The TiB₂ grains are predominantly threeto ten microns in diameter. The Ti (C,O,N) phase occurs as occlusionsless than one micron in size within the TiB₂ grains.

EXAMPLE XVI

A portion of the titanium diboride powder of Example V, Run B was usedto prepare two cold pressed and sintered cylindrical rods. The powderwas loaded into pressing molds and dies in dry nitrogen filled glovebags and pressed under a nitrogen atmosphere. Pressing was performedisostatically in a rubber mold at 20,000 and 30,000 pounds per squareinch (psi) respectively. The rods were buried in graphite powder in avacuum furnace and vacuum sintered. The furnace was heated to 2000° C.in 21/2 hours and held at that temperature for 25 minutes. The furnacewas turned off and the temperature decreased to 1625° C. in 15 minutes.After cooling, the rods were submitted for density measurements. The rodwhich was pressed at 20,000 psi had a density of 4.44 g/cc. The rodwhich was pressed at 30,000 psi had a density of 4.40 g/cc. Theaforesaid densities are 98 and 97 percent of theoretical based on atheoretical TiB₂ density of 4.51 grams/cc.

EXAMPLE XVII

A portion of the titanium diboride powder of Example V, Run B was hotpressed at 1600° C. and at 4500 pounds per square inch (psi). The timeinterval at 1600° C. was 30 minutes. The hot pressed specimen had adensity of 4.14 g/cc. (about 92 percent of the theoretical density ofTiB₂) and a transverse rupture strength of 34,000 psi.

A further portion of the titanium diboride powder of Example V, Run Bwas hot pressed at 1750° C. and at 5000 psi for 10 minutes. The hotpressed specimen had a density of 4.46 g/cc. (about 98 percenttheoretical) and a transverse rupture strength of 47,000 psi.

The following example illustrates a further preparation of titaniumdiboride powder in accordance with the present invention.

EXAMPLE XVIII

The apparatus and general procedure of Example VI was used except thattitanium tetrachloride in the amount of 72.2 grams per minute and 15SCFH of hydrogen were introduced into the reactor through the top slotof the reactant mixer assembly means. 1.26 Grams per minute of1,1,2-trichloroethane, 45 SCFH of hydrogen and 20 SCFH of hydrogenchloride were introduced through the middle slot and boron trichloridein an amount calculated to represent a 10 percent stoichiometric excess(basis the titanium tetrachloride) and 8 SCFH of argon were introducedthrough the bottom slot of the reactant mixer assembly. The titaniumdiboride powder product recovered had a B.E.T. surface area of 11.5square meters per gram and was found to contain about 31.6 percentboron, 0.08 percent chlorine, 0.19 percent oxygen and 1 percent totalcarbon.

EXAMPLE XIX

Submicron titanium diboride powder was prepared by vapor phase reactionof titanium tetrachloride and boron trichloride in the presence ofhydrogen and in the substantial absence of oxygen using plasma archeater and reactor apparatus analogous to that described in Example VI.Hydrogen chloride with hydrogen carrier gas was introduced into thereactor through the top slot of the reactant mixer assembly means.Reactant streams of titanium tetrachloride with hydrogen carrier gas,and boron trichloride with argon carrier gas were introduced through themiddle and bottom slots respectively of the mixer assembly. The carbonsource reactant, i.e., 1,1,2-trichloroethane, which had been used inExample XVIII, was not used. The reactions were introduced at a ratecalculated to produce theoretically five (5) pounds per hour of titaniumdiboride. After operating in the above manner for a period to purgecarbon or carbon species from the reactor and collection system, apowder product was collected for testing. The powder collected had asurface area of 7.4 m² /gram and the following average chemicalanalysis: Boron--30.7 percent, Total carbon--0.06 percent, Freecarbon--0.07 percent, Chlorine--0.16 percent, and Oxygen--0.06 percent.

A portion of the aforesaid powder was isostatically pressed at about20,000 pounds per square inch (psi) into a finger-like test slug andsintered for 1 hour at 2050° C. The density of the sintered slug was86.5 percent of theoretical. A second test slug prepared from thispowder and sintered for 1 hour at 2050° C. had a density of 82.5 percentof theoretical. The total carbon content of the first sintered slug was0.09 percent.

Examination of a polished metallographic section of the first sinteredslug evidenced a microstructure having a level of porosity too high toestimate by the ASTM method. The pore size was roughly equal to thegrain size. The grain size was non-uniform. The slug had a fine-grainedcasing followed by a coarse-grained zone and a finer-grained interior.The grains in the course zone were predominantly in the range of 3-25microns; while the grains in the finer zones were predominantly in therange of 1-12 microns.

A portion of the aforesaid powder was coated with about 2 weight percentGulfwax 20 and a finger-like test slug prepared and sintered in themanner described for the uncoated powder. Oxygen analysis of thewax-coated powder was reported as 0.17 percent. The sintered density ofthe slug was 97.5 percent of theoretical. The average total carbonanalysis of the sintered slug was 0.19 percent. Examination of apolished metallographic section of this slug evidenced an ASTM porosityof A-4, B-1. Intergranular porosity ranged in size from less than 1 to 4microns. The grain size was essentially fine with a predominant range of2-18 microns.

Hollow cylinders having a length to diameter ratio of about 0.9 wereprepared by isostatically pressing a portion of the wax coated powder ofthis Example at about 15,500 psi. The ends of the cylinders were trimmedand a piece of each end sintered simultaneously with the cylinders for 1hour at 2200° C. The top and bottom of one cylinder had densities of98.4 and 98.2 percent of theoretical respectively; the top and bottom ofa second cylinder had densities of 97.8 and 97.5 percent of theoreticalrespectively. Duplicate total carbon and oxygen analysis of the firstcylinder ends were reported as 0.22, 0.20 and 0.025, 0.015 percentrespectively. The trimmed ends of the cylinders are regarded as beingrepresentative of the properties of the cylinder and provide a methodfor analyzing chemically the sintered cylinder without destruction ofthe cylinder.

Examination of a polished metallographic section of the top of the firstcylinder evidenced an ASTM porosity of A-4, B-4. The grain size wasnon-uniform. A fine grained exterior about 1200 microns thick with acoarse grained interior was observed. The fine and coarse grains rangedin size from about 2-15 and 10-50 microns respectively. A system ofmicrocracks surrounding grains was discernible, as was an occluded TiCphase.

EXAMPLE XX

The process described in Example XIX was continued and the titaniumdiboride powder collected had a surface area of 6.5 m² /gram and thefollowing average chemical analysis: Total carbon--0.04 percent, Freecarbon--0.01 percent, Chlorine--0.18 percent, Oxygen--0.20 percent, andBoron--30.9 percent. A portion of the aforesaid powder was isostaticallypressed at about 20,000 psi into a finger-like test slug and sinteredfor 1 hour at 2050° C. The sintered density of this slug was 97.0percent of theoretical. The average total carbon analysis of thesintered slug was 0.10 percent.

Examination of a polished metallographic section of the slug evidencedan ASTM porosity of A-4, B-1. The grain size was fairly uniform. Themaximum grain size was about 18 microns; the minimum grain size wasabout 1 micron; and the predominant grain size ranged from 2-12 microns.Grain boundary pores were primarily 1-4 microns in size.

A portion of the aforesaid powder was coated with about 2 weight percentGulfwax 20 and a finger-like test slug prepared and sintered in themanner described for the uncoated powder. Oxygen analysis of thewax-coated powder was reported as 0.16 percent. The sintered density ofthe slug was 98.7 percent theoretical. The average total carbon analysisof the sintered slug was 0.17 percent.

Examination of a polished metallographic section of the slug evidencedan ASTM porosity of A-3, B-1. The grain size was fairly uniform. Themaximum grain size was about 35 microns; the minimum grain size wasabout 2 microns; and the predominant grain size ranged from 3-25microns. Compared to the slug prepared from the uncoated powder, thegrains were slightly more coarse; porosity was lower and finer, thepores being essentially submicron and primarily intragranular.

EXAMPLE XXI

The process described in Example XX was continued and the titaniumdiboride powder collected had a surface area of 5.8 m² /gram and thefollowing average chemical analysis: Boron--30.7 percent, Totalcarbon--0.03 percent, Free carbon--<0.01 percent, Chlorine--0.15percent, and Oxygen--0.11 percent. A portion of the aforesaid powder wasisostatically pressed at about 20,000 psi into a finger-like test slugand sintered for 1 hour at 2050° C. The sintered density of this slugwas 96.9 percent of theoretical and its average total carbon analysisreported as 0.08 percent.

Examination of a polished metallographic section of the slug evidencedan ASTM porosity of A-6, B-0, i.e., fine, submicron porosity. The grainsize was considered fine. The maximum grain size was about 10 microns;the minimum grain size was less than 1; and the predominant grain sizeranged from 1-8 microns. Some defects were observed as were some pores1-2 microns in size.

A portion of the aforesaid powder was coated with about 2 weight percentGulfwax 20 and a finger-like test slug prepared and sintered in themanner described for the uncoated powder. Oxygen analysis of thewax-coated powder was reported as 0.12 percent. The sintered density ofthe slug was 97.9 percent of theoretical and duplicate total carbon andoxygen analyses of the slug were reported as 0.13, 0.13 and 0.012, 0.015percent respectively.

Examination of a polished metallographic section of the slug evidencedas ASTM porosity of A-4, B-1. The A type porosity ranged from submicronto about 2 microns in size; but was primarily substantially below 2microns--although not completely submicron. The A type pores were mixedbetween intergranular (open) and intragranular (closed) pores. The grainsize was moderate, ranging from a maximum of 20 microns to a minimum of2 microns. The predominant grain size ranged between 3 and 15 microns. Alow to moderate level of microcracks was discernible at the grainboundaries as was a grey TiC phase. The TiB₂ grains were equant anddepicted a mosaic of equidimensional grains with contiguous grainboundaries.

A portion of the aforesaid wax coated powder was used to prepare ahollow cylinder in the manner described in Example XIX. The sinteredtrimmed top and bottom of the cylinder had densities of 97.1 and 98.3percent of theoretical.

EXAMPLE XXII

The process described in Example XXI was continued except that1,1,2-trichloroethane was introduced into the reactor at a rate of 0.2grams per minute. The titanium diboride powder collected had a surfacearea of 7.5 m² /gram and the following average chemical analysis:Boron--30.7 percent, Total carbon--0.13 percent, Free carbon--0.04percent, Chlorine--0.18 percent, and Oxygen--0.08 percent. A portion ofthe aforesaid powder was isostatically pressed at about 20,000 psi intoa finger-like test slug and sintered for 1 hour at 2050° C. The sintereddensity of this slug was 98.3 percent of theoretical and its averagetotal carbon and oxygen analysis reported as 0.17 and 0.013 percentrespectively.

Examination of a polished metallographic section of the slug evidencedan ASTM porosity of A-4, B-2. The grain size was moderate, rangingpredominantly from 3-25 microns. Microcracks were discernible at thegrain boundaries as was a grey TiC phase.

A portion of the aforesaid powder was coated with about 2 weight percentGulfwax 20 and a finger-like test slug prepared and sintered in themanner described for the uncoated powder. Oxygen analysis of thewax-coated powder was reported as 0.14 percent. The sintered density ofthe slug was 98.0 percent of theoretical.

EXAMPLE XXIII

The process described in Example XXII was continued except that the feedrate of 1,1,2-trichloroethane was increased to 0.46 grams per minute.The titanium diboride powder collected had a surface area of 7.3 m²/gram and the following average chemical analysis: Boron--30.6 percent,Total carbon--0.16 percent, Free carbon--0.08 percent, Chlorine--0.20percent and Oxygen--0.08 percent. A portion of the aforesaid powder wasisostatically pressed at about 20,000 psi into a finger-like test slugand sintered for 1 hour at 2050° C. The sintered density of this slugwas 98.8 percent of theoretical and duplicate total carbon and oxygenanalyses of the slug were reported as 0.196, 0.162 and 0.022, 0.013percent respectively.

Examination of a polished metallographic section of the slug evidencedan ASTM porosity of A-3, B-2. The grain size was moderate, ranging from3-25 microns. The microstructure was similar to that of the slugprepared from the uncoated powder of Example XXII in regard to grainsize, microcracks and the grey TiC phase.

A portion of the aforesaid powder was coated with about 2 weight percentGulfwax 20 and a finger-like test slug prepared and sintered in themanner described for the uncoated powder. Oxygen analysis of thewax-coated powder was reported as 0.09 percent. The sintered density ofthe slug was 98.8 percent of theoretical.

The data of Examples XIX-XXIII shows that titanium diboride powder ofthe present invention can be cold pressed and sintered into shapedarticles of high density. It shows also that a small amount of carbon ispicked up by the shaped article from the sintering furnace and/or thewax binder during sintering.

The microstructures described in connection with Examples XIX-XXIII weredetermined by optical microscopy of polished metallographic sections.Photomicrographs were taken at 200× and 1500× magnification factorsbefore etching to reveal porosity and microcracks, and after etching toshow grain size and other textural characteristics.

Examination of the microstructures described in Examples XIX-XXIII showthat the grain size of TiB₂ articles having a reported total carboncontent of from 0.13 to 0.19 percent is relatively fine when thearticles are sintered at 2050° C. for one hour. The grain size ispredominantly in the range from 2-25 microns. The microstructures arehomogeneous and are characterized by relatively uniform grain size. Thegrains are equant with contiguous grain boundaries. The porosity of sucharticles is also low, e.g., ASTM porosity of A-4, B-1, and isdistributed primarily as intragranular pores with some intergranularpores. The pore size is predominantly submicron but ranges from lessthan 1 to about 4 microns in size. A low to moderate level ofmicrocracks along grain boundaries is discernible in suchmicrostructures. These cracks are submicron in width and are short-rangein length, i.e., they generally have lengths about equivalent to theTiB₂ grain diameters.

When the carbon level of the TiB₂ articles is reduced to about 0.08 to0.10 percent, the microstructures of articles sintered at 2050° C. forone hour generally show increased porosity and larger pores as well as achange in distribution from intragranular to intergranular locations. Inaddition, grain size tends to be variable and specimens may be zonedwith coarse and fine grain size zones.

The microstructure of a TiB₂ article having a reported total carboncontent of 0.19 percent was more coarse when sintered at 2200° C. forone hour than when sintered at 2050° C. for one hour and may have beenzoned. The grain size in the finer zone may range from 2-15 microns andin the coarser zone may range from 10-50 microns. The porosity in thisarticle, except for Type B porosity, is primarily intragranular.Microcracks along grain boundaries were well developed and conspiciousat 1500× magnification in the unetched microstructure.

Although the present process has been described with reference tospecific details of certain embodiments thereof, it is not intended thatsuch details should be regarded as limitations upon the scope of theinvention except as and to the extent that they are included in theaccompanying claims.

I claim:
 1. A process for preparing submicron refractory metal diboridepowder of the metals titanium, zirconium and hafnium, which powder iscapable of being cold formed and sintered to a density of at least 90percent of its theoretical density, by gas phase reaction of the halideof the corresponding metal and boron source reactants in the presence ofhydrogen in a reactor, which comprises projecting a plasma heatedhydrogen gas stream into a reaction zone in the reactor, introducingsubstantially pure gaseous metal halide and boron source reactants intosaid reaction zone, the temperature of the reactants so introduced beingbelow the boride forming reaction temperature in the reaction zone andthe heat content of the hydrogen gas stream and reactants beingsufficient to establish refractory metal boride forming reactiontemperatures in said reaction zone, reacting said metal halide and boronsource reactants in the reaction zone in the substantial absence ofoxygen and a source of carbon, and removing solid, submicron refractorymetal boride powder from the reactor, at least 90 percent of theparticles of which have a nominal sectional diameter of less than onemicron, said particles having a number median particle size of betweenabout 0.08 and 0.6 microns and being tabular to equidimensionalhexagonal crystals with well developed faces.
 2. The process of claim 1wherein the metal halide reactant is the tetrachloride of titanium,zirconium or hafnium.
 3. The process of claim 1 or 2 wherein the boronsource reactant is boron trichloride.
 4. The process of claim 1 whereinthe reactor is a recirculating-type reactor.
 5. The process of claim 3wherein the plasma heating means is a plasma arc heater.
 6. A processfor preparing submicron titanium diboride powder, which powder iscapable of being cold formed and sintered to a density of at least 90percent of the theoretical density for titanium diboride, by gas phasereaction of titanium tetrachloride and boron trichloride reactants inthe presence of hydrogen in a reactor, which comprises heating hydrogenin plasma arc heater means and projecting the heated hydrogen as a gasstream into a reaction zone in the reactor, introducing substantiallypure gaseous titanium tetrachloride and boron trichloride reactants intosaid reaction zone, the temperature of the reactants so introduced beingbelow the titanium diboride reaction temperature in the reaction zoneand the heat content of said heated hydrogen and reactants beingsufficient to establish titanium diboride forming temperatures in thereaction zone, reacting said reactants in the substantial absence ofoxygen and a source of carbon, and removing solid, submicron titaniumdiboride powder from the reactor, at least 90 percent of the particlesof which have a nominal sectional diameter of less than one micron, saidparticles having a number median particle size of between 0.08 and 0.6micron and being tabular to equidimensional hexagonal crystals with welldeveloped faces, said powder having a surface area of between about 3and 35 square meters per gram and containing less than 0.4 weightpercent metal impurities and less than 0.1 weight percent carbon.
 7. Theprocess of claim 6 wherein the mole ratio of boron trichloride totitanium tetrachloride is from 1.8:1 to 3:1.
 8. The process of claim 6wherein the principal source of heat for the reaction is the heatedhydrogen gas stream.
 9. The process of claim 6 wherein the titaniumdiboride powder removed from the reactor has a surface area of betweenabout 4 and 15 square meters per gram, and the number median particlesize of said particles are between about 0.1 and about 0.5 micron. 10.The process of claims 6 or 9 wherein the solid titanium diboride powderremoved from the reactor contains less than 0.25 weight percent oxygen.11. A titanium diboride article of manufacture having a density of atleast 90 percent of the theoretical density of titanium diboride and acarbon content of less than 0.1 weight percent, which article isprepared by hot pressing, hot isostatic pressing, or cold forming andsintering submicron titanium diboride powder having a surface area ofbetween about 3 and 25 square meters per gram and containing less than0.4 weight percent metal impurities and less than 0.1 weight percentcarbon, the nominal sectional diameter of at least 90 percent of thetitanium diboride particles of said powder being less than one micron,said particles being tabular to equidimensional hexagonal crystalshaving well developed faces and a number median article size of betweenabout 0.08 and 0.6 microns.
 12. The article of claim 11 wherein thesurface area of the titanium diboride powder is between 3 and 15 squaremeters per gram.
 13. The article of claim 11 wherein the surface area ofthe titanium diboride powder is between 5 and 10 square meters per gram.14. The article of claims 11, 12, or 13 wherein the titanium diboridepowder contains less than 0.25 weight percent oxygen.
 15. The article ofclaim 14 wherein the article has a density of from 96 to 98 percent. 16.The article of claims 11, 12, or 13, wherein the titanium diboridepowder contains less than 0.3 weight percent metal impurities, less than0.20 weight percent oxygen, less than 0.15 weight percent halogen asdetermined by X-ray spectrographic analysis, and the number medianparticle size of the particles is between 0.1 and 0.5 microns.
 17. Atitanium diboride article of manufacture having a density of at least 90percent of the theoretical density of titanium diboride and a carboncontent of from 0.1 to 0.2 weight percent, which article is prepared byhot pressing, hot isostatic pressing or cold forming and sinteringsubmicron titanium diboride powder having a surface area of betweenabout 3 and 25 square meters per gram and containing less than 0.4weight percent metal impurities and less than 0.1 weight percent carbon,the nominal sectional diameter of at least 90 percent of the titaniumdiboride particles of said powder being less than one micron, saidparticles being tabular to equidimensional hexagonal crystals havingwell developed faces and a number of median particle size of betweenabout 0.08 and 0.6 microns.
 18. The article of claim 17 wherein thesurface area of the titanium diboride powder is between 3 and 15 squaremeters per gram.
 19. The article of claim 17 wherein the surface area ofthe titanium diboride powder is between 5 and 10 square meters per gram.20. The article of claims 17, 18, or 19 wherein the titanium diboridepowder contains less than 0.25 weight percent oxygen.
 21. The article ofclaim 20 wherein the article has a density of from 96 to 98 percent. 22.The article of claims 17, 18, or 19 wherein the titanium diboride powdercontains less than 0.3 weight percent metal impurities, less than 0.20weight percent oxygen, less than 0.15 weight percent halogen asdetermined by X-ray spectrographic analysis, and the number medianparticle size of the particles is between 0.1 and 0.5 microns.